Account Type:
| Model | Pickup Distance | Amount | Available Time | |
|---|---|---|---|---|
|
|
Car T400 | 10km | $900 | 10am - 5pm |
|
|
Car T400 | 10km | $900 | 10am - 5pm |
|
|
Car T400 | 10km | $900 | 10am - 5pm |
|
|
Car T400 | 10km | $900 | 10am - 5pm |
|
|
Car T400 | 10km | $900 | 10am - 5pm |
|
|
Car T400 | 10km | $900 | 10am - 5pm |


--- layout: default title: Sign up signup: class="active" ---
Head on over to an Athena computer and run
$ add scripts; signup-sqlin a terminal. Then, follow the on screen instructions, and you’ll be ready to go!
You can find out your username and password by running
$ cat ~/.my.cnf
Now that you’re registered, sign in to manage your databases!
You can also sign up group accounts (lockers) you have administrator privileges on. If you’re using Scripts autoinstallers, they’ll automatically sign up for a sql.mit.edu account if needed.
athena.dialup
has you covered. Log in with your MIT Athena account and run
the commands there. You can also use ssh
<EMAIL_ADDRESS>from a Mac or Linux
computer. If you’re using Windows, you can download and
use PuTTY.
If you run into any problems, or have any questions, send an email to<EMAIL_ADDRESS>We’re happy to help out!


public class Constants
extends java.lang.Object
| Modifier and Type | Field and Description |
|---|---|
static java.lang.String |
ACTION_COORDS_READY |
static java.lang.String |
ACTION_GET_OFF |
static java.lang.String |
ACTION_NEXT_GEOFENCE_REACHED
Action/Intent when GeoFence transition is detected by LocationClient.
|
static java.lang.String |
ACTION_NEXT_WAYPOINT
Action strings of Intents.
|
static java.lang.String |
ACTION_ROUTE_EVENT |
static java.lang.String |
ACTION_ROUTE_EVENT_EXTRA_MESSAGE |
static java.lang.String |
ERROR |
static int |
GEOFENCE_RADIUS
Radius of GeoFence regions (meters?).
|
static boolean |
getCoordsForNoStopCode
Whethre to try to obtain GPS coordinates for waypoints whose
HSL stopCode is unavailable (such as walking waypoints).
|
static boolean |
isDevelopmentMode
Debug/Development mode indicator.
|
static int |
LANG_EN |
static int |
LANG_FI
Language codes.
|
static int |
LANG_SV |
static double |
NO_COORD
Magic Latitude/Longitude number.
|
static java.lang.String |
NO_STOP_CODE
Magic stopCode for those stops without one.
|
static int |
NOTIFICATION_ID
id of the notification
|
static java.lang.String |
PEBBLE_APP_UUID
The UUID of our Pebble smart watch application.
|
static java.lang.String |
ROUTE_STOPPED |
static int |
SERVICE_LOCATION_AND_TIME |
static int |
SERVICE_LOCATION_ONLY |
static int |
SERVICE_TIME_ONLY
Tracker service types.
|
static long |
TEST_TIME
Interval of time-based events in development mode.
|
static boolean |
useWallClock
Whether time-based service(s) should use wall clock or toy time.
|
static int |
VIBRATION_TIME
how long the phone vibrates on alerts.
|
static int |
VIBRATOR_DURATION
Duration to vibrate on certain events.
|
| Modifier | Constructor and Description |
|---|---|
private |
Constants()
Prevent instantiation of this class.
|
public static final boolean isDevelopmentMode
public static final boolean useWallClock
public static final boolean getCoordsForNoStopCode
public static final int GEOFENCE_RADIUS
public static final java.lang.String ACTION_NEXT_WAYPOINT
public static final java.lang.String ACTION_GET_OFF
public static final java.lang.String ACTION_ROUTE_EVENT
public static final java.lang.String ACTION_ROUTE_EVENT_EXTRA_MESSAGE
public static final java.lang.String ACTION_COORDS_READY
public static final java.lang.String ACTION_NEXT_GEOFENCE_REACHED
public static final java.lang.String ROUTE_STOPPED
public static final java.lang.String ERROR
public static final long TEST_TIME
public static final int VIBRATOR_DURATION
public static final int SERVICE_TIME_ONLY
public static final int SERVICE_LOCATION_ONLY
public static final int SERVICE_LOCATION_AND_TIME
public static final java.lang.String NO_STOP_CODE
public static final double NO_COORD
public static final java.lang.String PEBBLE_APP_UUID
appinfo.json
,
Constant Field Valuespublic static final int LANG_FI
public static final int LANG_SV
public static final int LANG_EN
public static final int NOTIFICATION_ID
public static final int VIBRATION_TIME


0. ROS 2 publisher¶
| Source code | |
|---|---|
vadd_publisher |
|
| publisher | vadd_publisher.cpp |
doublevadd_publisher |
|
| publisher | doublevadd_publisher.cpp |
This first example presents a trivial vector-add ROS 2 publisher, which adds two vector inputs in a loop, and tries to publish the result at 10 Hz. The ROS 2 package runs in the scalar processors (the CPUs). Step-by-step, the process is documented, walking through the different actions required to run the ROS 2 package in hardware, leveraging KRS capabilities. Afterwards, a slighly modified version of the publisher is presented which has additional computation demands. With these modifications, it becomes clear how the publisher isn’t able to meet the publication goal anymore, which motivates the use of hardware acceleration.
The ultimate objective of this example is to generate a simple ROS 2-centric example that creates a CPU baseline to understand the value of hardware acceleration and how KRS facilitates it. Next examples will build upon this one.
Important
The examples assume you’ve already installed KRS. If not, refer to install.
Note
Learn ROS 2 before trying this out first.
vadd_publisher
¶
Prepare the environment and fetch the example¶
We start by preparing the environment and fetching the source code of the example into our KRS workspace:
$ cd ~/krs_ws # head to your KRS workspace
# prepare the environment
$ source /tools/Xilinx/Vitis/2020.2/settings64.sh # source Xilinx tools
$ source /opt/ros/foxy/setup.bash # Sources system ROS 2 installation
$ export PATH="/usr/bin":$PATH # FIXME: adjust path for CMake 3.5+
# fetch the source code of examples
$ git clone https://github.com/ros-acceleration/acceleration_examples src/acceleration_examples
# build the workspace
$ colcon build --merge-install # about 2 mins
# source the workspace as an overlay
$ source install/setup.bash
Inspecting the ROS 2 publisher¶
The publisher is a CPU-based average one. The source code of the publisher has been split between the vadd
function (vadd.cpp) and the rest (vadd_publisher.cpp) for simplicity.
The vadd
(vector-add) function is as follows:
/*
____ ____
/ /\/ /
/___/ \ / Copyright (c) 2021, Xilinx®.
\ \ \/ Author: Víctor Mayoral Vilches<EMAIL_ADDRESS>\ \
/ /
/___/ /\
\ \ / \
\___\/\___\
Inspired by the Vector-Add example.
See https://github.com/Xilinx/Vitis-Tutorials/blob/master/Getting_Started/Vitis
*/
#define DATA_SIZE 4096
// TRIPCOUNT identifier
const int c_size = DATA_SIZE;
extern "C" {
void vadd(
const unsigned int *in1, // Read-Only Vector 1
const unsigned int *in2, // Read-Only Vector 2
unsigned int *out, // Output Result
int size // Size in integer
)
{
for (int i = 0; i < size; ++i) {
#pragma HLS loop_tripcount min = c_size max = c_size
out[i] = in1[i] + in2[i];
}
}
}
The loop_tripcount
is for analysis only and the pragma doesn’t impact the function logic in any way. Instead, it allows HLS to identify how many iterations are expected in the loop to make time estimations. This will come handy if we want to run synthesis tests to estimate the timing it’ll take for this function to run on a dedicated circuit in the FPGA.
Building, creating the raw image and running in hardware¶
Let’s build the example, create a raw SD card image and run it in the KV260
reference development platform. First, let’s select the firmware for the target hardware, KV260:
$ colcon acceleration select kv260
To verify that we indeed that the right firmware selected, look for the one marked with a “*” at the end of its name:
$ colcon acceleration list
kv260*
Let’s now build the package targeting the KV260:
$ colcon build --build-base=build-kv260 --install-base=install-kv260 --merge-install --mixin kv260 --packages-select ament_vitis vadd_publisher
Let’s now create a raw disk image for the SD card with PetaLinux’s rootfs, a vanilla Linux 5.4.0 kernel and the ROS 2 overlay workspace we’ve just created for the KV260:
$ colcon acceleration linux vanilla --install-dir install-kv260
We’re now ready to run it on hardware. For that, we need to flash the ~/krs_ws/acceleration/firmware/select/sd_card.img
file into the SD card. One quick way to do it is as follows:
# first, find out where your SD card has been mapped, in my case, /dev/rdisk2
$ sudo diskutil umount /dev/rdisk2s1 # umount mounted partition
$ pv <your-path-to>/krs_ws/acceleration/firmware/select/sd_card.img | sudo dd of=/dev/rdisk2 bs=4m # dd the image
There are other methods. If you need help doing so, check out these instructions for different OSs. Make sure to flash ~/krs_ws/acceleration/firmware/select/sd_card.img
we just generated, and not some other image.
Once flashed, connect the board to the computer via its USB/UART/JTAG FTDI adapter and power it on. Then, launch your favority serial console (e.g. sudo tio /dev/ttyUSB1
). You should get to the following prompt where you can use the petalinux
username and define your own password:
PetaLinux 2020.2.2 xilinx-k26-starterkit-2020_2.2 ttyPS0
xilinx-k26-starterkit-2020_2 login:
Then, we can launch the example (feel free to split the actions below in various terminals, for convenience byobu
allows to run it all in one):
$ source /usr/bin/ros_setup.bash # source the ROS 2 installation
$ . /ros2_ws/local_setup.bash # source the ROS 2 overlay workspace we just
# created.
You should see that the ROS 2 publisher successfully publishes at approximately 10 Hz.
Bonus: Getting a time intuition of a vadd
acceleration kernel¶
What if instead of running the vadd
in the scalar CPUs, we create a specific hardware circuit with the FPGA that runs this function? What’d would be the associated timing? In other words, a robot chip for the vadd
computation.
Though going all the way down to implementing the harware cirtuit for vadd
is beyond the scope of this example, we can get a quick intuition with HLS capabilities integrated in KRS. Let’s get such intuition:
$ colcon acceleration hls --run --synthesis-report vadd_publisher
Found Tcl script "project_vadd_publisher.tcl" for package: vadd_publisher
Executing /home/xilinx/krs_ws/build-kv260/vadd_publisher/project_vadd_publisher.tcl
Project: project_vadd_publisher
Path: /home/xilinx/krs_ws/build-kv260/vadd_publisher/project_vadd_publisher
- Solution: solution_4ns
- C Simulation: Pass
- C Synthesis: Run
- C/RTL Co-simulation: Not Run
- Export:
- IP Catalog: Not Run
- System Generator: Not Run
- Export Evaluation: Not Run
- Synthesis report: /home/xilinx/krs_ws/build-kv260/vadd_publisher/project_vadd_publisher/solution_4ns/syn/report/vadd_csynth.rpt
================================================================
== Vitis HLS Report for 'vadd'
================================================================
* Date: Sun Aug 22 18:24:43 2021
* Version: 2020.2.2 (Build 3118627 on Tue Feb 9 05:13:49 MST 2021)
* Project: project_vadd_publisher
* Solution: solution_4ns (Vitis Kernel Flow Target)
* Product family: zynquplus
* Target device: xck26-sfvc784-2LV-c
================================================================
== Performance Estimates
================================================================
+ Timing:
* Summary:
+--------+---------+----------+------------+
| Clock | Target | Estimated| Uncertainty|
+--------+---------+----------+------------+
|ap_clk | 4.00 ns| 2.920 ns| 1.08 ns|
+--------+---------+----------+------------+
+ Latency:
* Summary:
+---------+---------+----------+-----------+-----+------+---------+
| Latency (cycles) | Latency (absolute) | Interval | Pipeline|
| min | max | min | max | min | max | Type |
+---------+---------+----------+-----------+-----+------+---------+
| 2| 8334| 8.000 ns| 33.336 us| 3| 8335| none|
+---------+---------+----------+-----------+-----+------+---------+
+ Detail:
* Instance:
N/A
* Loop:
+-------------------+---------+---------+----------+-----------+-----------+------+----------+
| | Latency (cycles) | Iteration| Initiation Interval | Trip | |
| Loop Name | min | max | Latency | achieved | target | Count| Pipelined|
+-------------------+---------+---------+----------+-----------+-----------+------+----------+
|- VITIS_LOOP_30_1 | 8264| 8264| 75| 2| 1| 4096| yes|
+-------------------+---------+---------+----------+-----------+-----------+------+----------+
================================================================
== Utilization Estimates
================================================================
* Summary:
+-----------------+---------+------+--------+--------+-----+
| Name | BRAM_18K| DSP | FF | LUT | URAM|
+-----------------+---------+------+--------+--------+-----+
|DSP | -| -| -| -| -|
|Expression | -| -| 0| 280| -|
|FIFO | -| -| -| -| -|
|Instance | 2| -| 796| 1068| -|
|Memory | -| -| -| -| -|
|Multiplexer | -| -| -| 472| -|
|Register | -| -| 623| 32| -|
+-----------------+---------+------+--------+--------+-----+
|Total | 2| 0| 1419| 1852| 0|
+-----------------+---------+------+--------+--------+-----+
|Available | 288| 1248| 234240| 117120| 64|
+-----------------+---------+------+--------+--------+-----+
|Utilization (%) | ~0| 0| ~0| 1| 0|
+-----------------+---------+------+--------+--------+-----+
...
The output will show you that the vadd
function with all its complexity can run with a 4 ns
clock (we could ask for other clocks) in a total of 33.336 us
. This result is completely deterministic from the vadd viewpoint, after all, the FPGA would create a specialized hardware cirtuit for the vadd
computations. For what concerns vadd, there’s no more deterministic execution than this. This is just a brief introduction, if you wish to learn more about HLS and KRS, head to the second example: 1. Hello Xilinx.
doublevadd_publisher
¶
We’ve seen that as a simple ROS 2 package, vadd_publisher
runs perfectly fine and meets the 10 Hz publishing objective. But what if the vadd
has bigger vectors, or has operations that involve many more iterations? This second section explores this with a more computationally expensive publisher, the doublevadd_publisher
. The source code of the new vadd
function is presented below:
/*
____ ____
/ /\/ /
/___/ \ / Copyright (c) 2021, Xilinx®.
\ \ \/ Author: Víctor Mayoral Vilches<EMAIL_ADDRESS>\ \
/ /
/___/ /\
\ \ / \
\___\/\___\
Inspired by the Vector-Add example.
See https://github.com/Xilinx/Vitis-Tutorials/blob/master/Getting_Started/Vitis
*/
#define DATA_SIZE 4096
// TRIPCOUNT identifier
const int c_size = DATA_SIZE;
extern "C" {
void vadd(
const unsigned int *in1, // Read-Only Vector 1
const unsigned int *in2, // Read-Only Vector 2
unsigned int *out, // Output Result
int size // Size in integer
)
{
for (int j = 0; j < size; ++j) { // stupidly iterate over
// it to generate load
#pragma HLS loop_tripcount min = c_size max = c_size
for (int i = 0; i < size; ++i) {
#pragma HLS loop_tripcount min = c_size max = c_size
out[i] = in1[i] + in2[i];
}
}
}
}
Note how instead of one for
loop, we now have two, simulating a more complex computation. Let’s try this out in hardware. Provided that the image was previously created, we just need to replace the ROS 2 workspace, the rest should be identical. You can do this in various ways (physically mounting the raw image in the SD card, scp
-ing the ROS 2 workspace, etc.).
Briefly, in the workstation:
# generate the workspace with doublevadd_publisher (if exists already, add to it)
$ colcon build --build-base=build-kv260 --install-base=install-kv260 --merge-install --mixin kv260 --packages-select ament_vitis doublevadd_publisher
# copy to rootfs in SD card, e.g.
$ scp -r install-kv260/* petalinux@<IP_ADDRESS>:/ros2_ws/
# Launch doublevadd_publisher
$ source /usr/bin/ros_setup.bash # source the ROS 2 installation
$ . /ros2_ws/local_setup.bash # source the ROS 2 overlay workspace we just
# created. Note it has been copied to the SD
# card image while being created.
$ ros2 topic hz /vector --window 10 &
$ ros2 run doublevadd_publisher doublevadd_publisher
...

This new publisher achieves only 2.2 Hz
, quite far from the 10 Hz
targeted. Using hardware acceleration, future examples will demonstrate how to build a custom compute pipeline that offloads computations to a kernel. If you wish to jump directly into hardware acceleration with doublevadd_publisher
, head to: 3. Offloading ROS 2 publisher.


DO NOT READ THIS FILE ON GITHUB, GUIDES ARE PUBLISHED ON https://guides.rubyonrails.org.
Active Record Associations
This guide covers the association features of Active Record.
After reading this guide, you will know:
- How to declare associations between Active Record models.
- How to understand the various types of Active Record associations.
- How to use the methods added to your models by creating associations.
Why Associations?
In Rails, an association is a connection between two Active Record models. Why do we need associations between models? Because they make common operations simpler and easier in your code. For example, consider a simple Rails
application that includes a model for authors and a model for books. Each author can have many books. Without associations, the model declarations would look like this:
class Author < ApplicationRecord
end
class Book < ApplicationRecord
end
Now, suppose we wanted to add a new book for an existing author. We'd need to do something like this:
@book = Book.create(published_at: Time.now, author_id: @author.id)
Or consider deleting an author, and ensuring that all of its books get deleted as well:
@books = Book.where(author_id: @author.id)
@books.each do |book|
book.destroy
end
@author.destroy
With Active Record associations, we can streamline these - and other - operations by declaratively telling Rails that there is a connection between the two models. Here's the revised code for setting up authors and books:
class Author < ApplicationRecord
has_many :books, dependent: :destroy
end
class Book < ApplicationRecord
belongs_to :
end
With this change, creating a new book for a particular author is easier:
@book = @author.books.create(published_at: Time.now)
Deleting an author and all of its books is much easier:
@author.destroy
To learn more about the different types of associations, read the next section of this guide. That's followed by some tips and tricks for working with associations, and then by a complete reference to the methods and options for associations in Rails.
The Types of Associations
Rails supports six types of associations:
Associations are implemented using macro-style calls, so that you can declaratively add features to your models. For example, by declaring that one model belongs_to
another, you instruct Rails to maintain Primary Key-Foreign Key information between instances of the two models, and you also get a number of utility methods added to your model.
In the remainder of this guide, you'll learn how to declare and use the various forms of associations. But first, a quick introduction to the situations where each association type is appropriate.
The belongs_to
Association
A belongs_to
association sets up a connection with another model, such that each instance of the declaring model "belongs to" one instance of the other model. For example, if your application includes authors and books, and each book can be assigned to exactly one author, you'd declare the book model this way:
class Book < ApplicationRecord
belongs_to :
end
NOTE: belongs_to
associations must use the singular term. If you used the pluralized form in the above example for the author
association in the Book
model and tried to create the instance by Book.create(authors: @author)
, you would be told that there was an "uninitialized constant Book::Authors". This is because Rails automatically infers the class name from the association name. If the association name is wrongly pluralized, then the inferred class will be wrongly pluralized too.
The corresponding migration might look like this:
class CreateBooks < ActiveRecord::Migration[6.0]
def change
create_table : do |t|
t.string :name
t.
end
create_table :books do |t|
t.belongs_to :
t.datetime :published_at
t.
end
end
end
When used alone, belongs_to
produces a one-directional one-to-one connection. Therefore each book in the above example "knows" its author, but the authors don't know about their books.
To setup a bi-directional association - use belongs_to
in combination with a has_one
or has_many
on the other model.
belongs_to
does not ensure reference consistency, so depending on the use case, you might also need to add a database-level foreign key constraint on the reference column, like this:
create_table :books do |t|
t.belongs_to :, foreign_key: true
# ...
end
The has_one
Association
A has_one
association indicates that one other model has a reference to this model. That model can be fetched through this association.
For example, if each supplier in your application has only one account, you'd declare the supplier model like this:
class Supplier < ApplicationRecord
has_one :account
end
The main difference from belongs_to
is that the link column supplier_id
is located in the other table:
The corresponding migration might look like this:
class CreateSuppliers < ActiveRecord::Migration[6.0]
def change
create_table :suppliers do |t|
t.string :name
t.
end
create_table :accounts do |t|
t.belongs_to :supplier
t.string :account_number
t.
end
end
end
Depending on the use case, you might also need to create a unique index and/or a foreign key constraint on the supplier column for the accounts table. In this case, the column definition might look like this:
create_table :accounts do |t|
t.belongs_to :supplier, index: { unique: true }, foreign_key: true
# ...
end
This relation can be bi-directional when used in combination with belongs_to
on the other model.
The has_many
Association
A has_many
association is similar to has_one
, but indicates a one-to-many connection with another model. You'll often find this association on the "other side" of a belongs_to
association. This association indicates that each instance of the model has zero or more instances of another model. For example, in an application containing authors and books, the author model could be declared like this:
class Author < ApplicationRecord
has_many :books
end
NOTE: The name of the other model is pluralized when declaring a has_many
association.
The corresponding migration might look like this:
class CreateAuthors < ActiveRecord::Migration[6.0]
def change
create_table : do |t|
t.string :name
t.
end
create_table :books do |t|
t.belongs_to :
t.datetime :published_at
t.
end
end
end
Depending on the use case, it's usually a good idea to create a non-unique index and optionally a foreign key constraint on the author column for the books table:
create_table :books do |t|
t.belongs_to :, index: true, foreign_key: true
# ...
end
The has_many :through
Association
A has_many :through
association is often used to set up a many-to-many connection with another model. This association indicates that the declaring model can be matched with zero or more instances of another model by proceeding through a third model. For example, consider a medical practice where patients make appointments to see physicians. The relevant association declarations could look like this:
class Physician < ApplicationRecord
has_many :appointments
has_many :patients, through: :appointments
end
class Appointment < ApplicationRecord
belongs_to :physician
belongs_to :patient
end
class Patient < ApplicationRecord
has_many :appointments
has_many :physicians, through: :appointments
end
The corresponding migration might look like this:
class CreateAppointments < ActiveRecord::Migration[6.0]
def change
create_table :physicians do |t|
t.string :name
t.
end
create_table :patients do |t|
t.string :name
t.
end
create_table :appointments do |t|
t.belongs_to :physician
t.belongs_to :patient
t.datetime :appointment_date
t.
end
end
end
The collection of join models can be managed via the has_many
association methods.
For example, if you assign:
physician.patients = patients
Then new join models are automatically created for the newly associated objects. If some that existed previously are now missing, then their join rows are automatically deleted.
WARNING: Automatic deletion of join models is direct, no destroy callbacks are triggered.
The has_many :through
association is also useful for setting up "shortcuts" through nested has_many
associations. For example, if a document has many sections, and a section has many paragraphs, you may sometimes want to get a simple collection of all paragraphs in the document. You could set that up this way:
class Document < ApplicationRecord
has_many :sections
has_many :paragraphs, through: :sections
end
class Section < ApplicationRecord
belongs_to :document
has_many :paragraphs
end
class Paragraph < ApplicationRecord
belongs_to :section
end
With through: :sections
specified, Rails will now understand:
@document.paragraphs
The has_one :through
Association
A has_one :through
association sets up a one-to-one connection with another model. This association indicates
that the declaring model can be matched with one instance of another model by proceeding through a third model.
For example, if each supplier has one account, and each account is associated with one account history, then the
supplier model could look like this:
class Supplier < ApplicationRecord
has_one :account
has_one :account_history, through: :account
end
class Account < ApplicationRecord
belongs_to :supplier
has_one :account_history
end
class AccountHistory < ApplicationRecord
belongs_to :account
end
The corresponding migration might look like this:
class CreateAccountHistories < ActiveRecord::Migration[6.0]
def change
create_table :suppliers do |t|
t.string :name
t.
end
create_table :accounts do |t|
t.belongs_to :supplier
t.string :account_number
t.
end
create_table :account_histories do |t|
t.belongs_to :account
t.integer :
t.
end
end
end
The has_and_belongs_to_many
Association
A has_and_belongs_to_many
association creates a direct many-to-many connection with another model, with no intervening model.
This association indicates that each instance of the declaring model refers to zero or more instances of another model.
For example, if your application includes assemblies and parts, with each assembly having many parts and each part appearing in many assemblies, you could declare the models this way:
class Assembly < ApplicationRecord
has_and_belongs_to_many :parts
end
class Part < ApplicationRecord
has_and_belongs_to_many :assemblies
end
The corresponding migration might look like this:
class CreateAssembliesAndParts < ActiveRecord::Migration[6.0]
def change
create_table :assemblies do |t|
t.string :name
t.
end
create_table :parts do |t|
t.string :part_number
t.
end
create_table :assemblies_parts, id: false do |t|
t.belongs_to :assembly
t.belongs_to :part
end
end
end
Choosing Between belongs_to
and has_one
If you want to set up a one-to-one relationship between two models, you'll need to add belongs_to
to one, and has_one
to the other. How do you know which is which?
The distinction is in where you place the foreign key (it goes on the table for the class declaring the belongs_to
association), but you should give some thought to the actual meaning of the data as well. The has_one
relationship says that one of something is yours - that is, that something points back to you. For example, it makes more sense to say that a supplier owns an account than that an account owns a supplier. This suggests that the correct relationships are like this:
class Supplier < ApplicationRecord
has_one :account
end
class Account < ApplicationRecord
belongs_to :supplier
end
The corresponding migration might look like this:
class CreateSuppliers < ActiveRecord::Migration[6.0]
def change
create_table :suppliers do |t|
t.string :name
t.
end
create_table :accounts do |t|
t.bigint :supplier_id
t.string :account_number
t.
end
add_index :accounts, :supplier_id
end
end
NOTE: Using t.bigint :supplier_id
makes the foreign key naming obvious and explicit. In current versions of Rails, you can abstract away this implementation detail by using t.references :supplier
instead.
Choosing Between has_many :through
and has_and_belongs_to_many
Rails offers two different ways to declare a many-to-many relationship between models. The first way is to use has_and_belongs_to_many
, which allows you to make the association directly:
class Assembly < ApplicationRecord
has_and_belongs_to_many :parts
end
class Part < ApplicationRecord
has_and_belongs_to_many :assemblies
end
The second way to declare a many-to-many relationship is to use has_many :through
. This makes the association indirectly, through a join model:
class Assembly < ApplicationRecord
has_many :manifests
has_many :parts, through: :manifests
end
class Manifest < ApplicationRecord
belongs_to :assembly
belongs_to :part
end
class Part < ApplicationRecord
has_many :manifests
has_many :assemblies, through: :manifests
end
The simplest rule of thumb is that you should set up a has_many :through
relationship if you need to work with the relationship model as an independent entity. If you don't need to do anything with the relationship model, it may be simpler to set up a has_and_belongs_to_many
relationship (though you'll need to remember to create the joining table in the database).
You should use has_many :through
if you need validations, callbacks, or extra attributes on the join model.
Polymorphic Associations
A slightly more advanced twist on associations is the polymorphic association. With polymorphic associations, a model can belong to more than one other model, on a single association. For example, you might have a picture model that belongs to either an employee model or a product model. Here's how this could be declared:
class Picture < ApplicationRecord
belongs_to :imageable, polymorphic: true
end
class Employee < ApplicationRecord
has_many :pictures, as: :imageable
end
class Product < ApplicationRecord
has_many :pictures, as: :imageable
end
You can think of a polymorphic belongs_to
declaration as setting up an interface that any other model can use. From an instance of the Employee
model, you can retrieve a collection of pictures: @employee.pictures
.
Similarly, you can retrieve @product.pictures
.
If you have an instance of the Picture
model, you can get to its parent via @picture.imageable
. To make this work, you need to declare both a foreign key column and a type column in the model that declares the polymorphic interface:
class CreatePictures < ActiveRecord::Migration[6.0]
def change
create_table :pictures do |t|
t.string :name
t.bigint :imageable_id
t.string :imageable_type
t.
end
add_index :pictures, [:imageable_type, :imageable_id]
end
end
This migration can be simplified by using the t.references
form:
class CreatePictures < ActiveRecord::Migration[6.0]
def change
create_table :pictures do |t|
t.string :name
t.references :imageable, polymorphic: true
t.
end
end
end
Self Joins
In designing a data model, you will sometimes find a model that should have a relation to itself. For example, you may want to store all employees in a single database model, but be able to trace relationships such as between manager and subordinates. This situation can be modeled with self-joining associations:
class Employee < ApplicationRecord
has_many :subordinates, class_name: "Employee",
foreign_key: "manager_id"
belongs_to :manager, class_name: "Employee", optional: true
end
With this setup, you can retrieve @employee.subordinates
and @employee.manager
.
In your migrations/schema, you will add a references column to the model itself.
class CreateEmployees < ActiveRecord::Migration[6.0]
def change
create_table :employees do |t|
t.references :manager, foreign_key: { to_table: :employees }
t.
end
end
end
Tips, Tricks, and Warnings
Here are a few things you should know to make efficient use of Active Record associations in your Rails applications:
- Controlling caching
- Avoiding name collisions
- Updating the schema
- Controlling association scope
- Bi-directional associations
Controlling Caching
All of the association methods are built around caching, which keeps the result of the most recent query available for further operations. The cache is even shared across methods. For example:
# retrieves books from the database
.books.load
# uses the cached copy of books
.books.size
# uses the cached copy of books
.books.empty?
But what if you want to reload the cache, because data might have been changed by some other part of the application? Just call reload
on the association:
# retrieves books from the database
.books.load
# uses the cached copy of books
.books.size
# discards the cached copy of books and goes back to the database
.books.reload.empty?
Avoiding Name Collisions
You are not free to use just any name for your associations. Because creating an association adds a method with that name to the model, it is a bad idea to give an association a name that is already used for an instance method of ::ActiveRecord::Base
. The association method would override the base method and break things. For instance, attributes
or connection
are bad names for associations.
Updating the Schema
Associations are extremely useful, but they are not magic. You are responsible for maintaining your database schema to match your associations. In practice, this means two things, depending on what sort of associations you are creating. For belongs_to
associations you need to create foreign keys, and for has_and_belongs_to_many
associations you need to create the appropriate join table.
Creating Foreign Keys for belongs_to
Associations
When you declare a belongs_to
association, you need to create foreign keys as appropriate. For example, consider this model:
class Book < ApplicationRecord
belongs_to :
end
This declaration needs to be backed up by a corresponding foreign key column in the books table. For a brand new table, the migration might look something like this:
class CreateBooks < ActiveRecord::Migration[6.0]
def change
create_table :books do |t|
t.datetime :published_at
t.string :book_number
t.references :
end
end
end
Whereas for an existing table, it might look like this:
class AddAuthorToBooks < ActiveRecord::Migration[6.0]
def change
add_reference :books, :
end
end
NOTE: If you wish to enforce referential integrity at the database level, add the foreign_key: true
option to the ‘reference’ column declarations above.
Creating Join Tables for has_and_belongs_to_many
Associations
If you create a has_and_belongs_to_many
association, you need to explicitly create the joining table. Unless the name of the join table is explicitly specified by using the :join_table
option, Active Record creates the name by using the lexical order of the class names. So a join between author and book models will give the default join table name of "authors_books" because "a" outranks "b" in lexical ordering.
WARNING: The precedence between model names is calculated using the <=>
operator for String
. This means that if the strings are of different lengths, and the strings are equal when compared up to the shortest length, then the longer string is considered of higher lexical precedence than the shorter one. For example, one would expect the tables "paper_boxes" and "papers" to generate a join table name of "papers_paper_boxes" because of the length of the name "paper_boxes", but it in fact generates a join table name of "paper_boxes_papers" (because the underscore '_' is lexicographically less than 's' in common encodings).
Whatever the name, you must manually generate the join table with an appropriate migration. For example, consider these associations:
class Assembly < ApplicationRecord
has_and_belongs_to_many :parts
end
class Part < ApplicationRecord
has_and_belongs_to_many :assemblies
end
These need to be backed up by a migration to create the assemblies_parts
table. This table should be created without a primary key:
class CreateAssembliesPartsJoinTable < ActiveRecord::Migration[6.0]
def change
create_table :assemblies_parts, id: false do |t|
t.bigint :assembly_id
t.bigint :part_id
end
add_index :assemblies_parts, :assembly_id
add_index :assemblies_parts, :part_id
end
end
We pass id: false
to create_table
because that table does not represent a model. That's required for the association to work properly. If you observe any strange behavior in a has_and_belongs_to_many
association like mangled model IDs, or exceptions about conflicting IDs, chances are you forgot that bit.
You can also use the method create_join_table
class CreateAssembliesPartsJoinTable < ActiveRecord::Migration[6.0]
def change
create_join_table :assemblies, :parts do |t|
t.index :assembly_id
t.index :part_id
end
end
end
Controlling Association Scope
By default, associations look for objects only within the current module's scope. This can be important when you declare Active Record models within a module. For example:
module MyApplication
module Business
class Supplier < ApplicationRecord
has_one :account
end
class Account < ApplicationRecord
belongs_to :supplier
end
end
end
This will work fine, because both the Supplier
and the Account
class are defined within the same scope. But the following will not work, because Supplier
and Account
are defined in different scopes:
module MyApplication
module Business
class Supplier < ApplicationRecord
has_one :account
end
end
module Billing
class Account < ApplicationRecord
belongs_to :supplier
end
end
end
To associate a model with a model in a different namespace, you must specify the complete class name in your association declaration:
module MyApplication
module Business
class Supplier < ApplicationRecord
has_one :account,
class_name: "MyApplication::Billing::Account"
end
end
module Billing
class Account < ApplicationRecord
belongs_to :supplier,
class_name: "MyApplication::Business::Supplier"
end
end
end
Bi-directional Associations
It's normal for associations to work in two directions, requiring declaration on two different models:
class Author < ApplicationRecord
has_many :books
end
class Book < ApplicationRecord
belongs_to :
end
Active Record will attempt to automatically identify that these two models share a bi-directional association based on the association name. In this way, Active Record will only load one copy of the Author
object, making your application more efficient and preventing inconsistent data:
irb> a = Author.first
irb> b = a.books.first
irb> a.first_name == b.author.first_name
=> true
irb> a.first_name = 'David'
irb> a.first_name == b.author.first_name
=> true
Active Record supports automatic identification for most associations with standard names. However, Active Record will not automatically identify bi-directional associations that contain a scope or any of the following options:
:through
:foreign_key
For example, consider the following model declarations:
class Author < ApplicationRecord
has_many :books
end
class Book < ApplicationRecord
belongs_to :writer, class_name: 'Author', foreign_key: 'author_id'
end
Active Record will no longer automatically recognize the bi-directional association:
irb> a = Author.first
irb> b = a.books.first
irb> a.first_name == b.writer.first_name
=> true
irb> a.first_name = 'David'
irb> a.first_name == b.writer.first_name
=> false
Active Record provides the :inverse_of
option so you can explicitly declare bi-directional associations:
class Author < ApplicationRecord
has_many :books, inverse_of: 'writer'
end
class Book < ApplicationRecord
belongs_to :writer, class_name: 'Author', foreign_key: 'author_id'
end
By including the :inverse_of
option in the has_many
association declaration, Active Record will now recognize the bi-directional association:
irb> a = Author.first
irb> b = a.books.first
irb> a.first_name == b.writer.first_name
=> true
irb> a.first_name = 'David'
irb> a.first_name == b.writer.first_name
=> true
Detailed Association Reference
The following sections give the details of each type of association, including the methods that they add and the options that you can use when declaring an association.
belongs_to
Association Reference
In database terms, the belongs_to
association says that this model's table contains a column which represents a reference to another table.
This can be used to set up one-to-one or one-to-many relations, depending on the setup.
If the table of the other class contains the reference in a one-to-one relation, then you should use has_one
instead.
Methods Added by belongs_to
When you declare a belongs_to
association, the declaring class automatically gains 6 methods related to the association:
association
association=(associate)
build_association(attributes = {})
create_association(attributes = {})
create_association!(attributes = {})
reload_association
In all of these methods, association
is replaced with the symbol passed as the first argument to belongs_to
. For example, given the declaration:
class Book < ApplicationRecord
belongs_to :
end
Each instance of the Book
model will have these methods:
=
NOTE: When initializing a new has_one
or belongs_to
association you must use the build_
prefix to build the association, rather than the association.build
method that would be used for has_many
or has_and_belongs_to_many
associations. To create one, use the create_
prefix.
association
The association
method returns the associated object, if any. If no associated object is found, it returns nil
.
@author = @book.
If the associated object has already been retrieved from the database for this object, the cached version will be returned. To override this behavior (and force a database read), call #reload_association
on the parent object.
@author = @book.
association=(associate)
The association=
method assigns an associated object to this object. Behind the scenes, this means extracting the primary key from the associated object and setting this object's foreign key to the same value.
@book. = @author
build_association(attributes = {})
The build_association
method returns a new object of the associated type. This object will be instantiated from the passed attributes, and the link through this object's foreign key will be set, but the associated object will not yet be saved.
@author = @book. (author_number: 123,
author_name: "John Doe")
create_association(attributes = {})
The create_association
method returns a new object of the associated type. This object will be instantiated from the passed attributes, the link through this object's foreign key will be set, and, once it passes all of the validations specified on the associated model, the associated object will be saved.
@author = @book. (author_number: 123,
author_name: "John Doe")
create_association!(attributes = {})
Does the same as create_association
above, but raises ::ActiveRecord::RecordInvalid
if the record is invalid.
Options for belongs_to
While Rails uses intelligent defaults that will work well in most situations, there may be times when you want to customize the behavior of the belongs_to
association reference. Such customizations can easily be accomplished by passing options and scope blocks when you create the association. For example, this association uses two such options:
class Book < ApplicationRecord
belongs_to :, touch: :books_updated_at,
counter_cache: true
end
The belongs_to
association supports these options:
:autosave
:class_name
:counter_cache
:dependent
:foreign_key
:primary_key
:inverse_of
:polymorphic
:touch
:validate
:optional
:autosave
If you set the :autosave
option to true
, Rails will save any loaded association members and destroy members that are marked for destruction whenever you save the parent object. Setting :autosave
to false
is not the same as not setting the :autosave
option. If the :autosave
option is not present, then new associated objects will be saved, but updated associated objects will not be saved.
:class_name
If the name of the other model cannot be derived from the association name, you can use the :class_name
option to supply the model name. For example, if a book belongs to an author, but the actual name of the model containing authors is Patron
, you'd set things up this way:
class Book < ApplicationRecord
belongs_to :, class_name: "Patron"
end
:counter_cache
The :counter_cache
option can be used to make finding the number of belonging objects more efficient. Consider these models:
class Book < ApplicationRecord
belongs_to :
end
class Author < ApplicationRecord
has_many :books
end
With these declarations, asking for the value of @author.books.size
requires making a call to the database to perform a COUNT(*)
query. To avoid this call, you can add a counter cache to the belonging model:
class Book < ApplicationRecord
belongs_to :, counter_cache: true
end
class Author < ApplicationRecord
has_many :books
end
With this declaration, Rails will keep the cache value up to date, and then return that value in response to the size
method.
Although the :counter_cache
option is specified on the model that includes
the belongs_to
declaration, the actual column must be added to the
associated (has_many
) model. In the case above, you would need to add a
column named books_count
to the Author
model.
You can override the default column name by specifying a custom column name in
the counter_cache
declaration instead of true
. For example, to use
count_of_books
instead of books_count
:
class Book < ApplicationRecord
belongs_to :, counter_cache: :count_of_books
end
class Author < ApplicationRecord
has_many :books
end
NOTE: You only need to specify the :counter_cache
option on the belongs_to
side of the association.
Counter cache columns are added to the containing model's list of read-only attributes through attr_readonly
.
:dependent
If you set the :dependent
option to:
:destroy
, when the object is destroyed,destroy
will be called on its associated objects.:delete
, when the object is destroyed, all its associated objects will be deleted directly from the database without calling theirdestroy
method.:destroy_async
: when the object is destroyed, an::ActiveRecord::DestroyAssociationAsyncJob
job is enqueued which will call destroy on its associated objects. Active Job must be set up for this to work.
WARNING: You should not specify this option on a belongs_to
association that is connected with a has_many
association on the other class. Doing so can lead to orphaned records in your database.
:foreign_key
By convention, Rails assumes that the column used to hold the foreign key on this model is the name of the association with the suffix _id
added. The :foreign_key
option lets you set the name of the foreign key directly:
class Book < ApplicationRecord
belongs_to :, class_name: "Patron",
foreign_key: "patron_id"
end
TIP: In any case, Rails will not create foreign key columns for you. You need to explicitly define them as part of your migrations.
:primary_key
By convention, Rails assumes that the id
column is used to hold the primary key
of its tables. The :primary_key
option allows you to specify a different column.
For example, given we have a users
table with guid
as the primary key. If we want a separate todos
table to hold the foreign key user_id
in the guid
column, then we can use primary_key
to achieve this like so:
class User < ApplicationRecord
self.primary_key = 'guid' # primary key is guid and not id
end
class Todo < ApplicationRecord
belongs_to :user, primary_key: 'guid'
end
When we execute @user.todos.create
then the @todo
record will have its
user_id
value as the guid
value of @user
.
:inverse_of
The :inverse_of
option specifies the name of the has_many
or has_one
association that is the inverse of this association.
class Author < ApplicationRecord
has_many :books, inverse_of: :
end
class Book < ApplicationRecord
belongs_to :, inverse_of: :books
end
:polymorphic
Passing true
to the :polymorphic
option indicates that this is a polymorphic association. Polymorphic associations were discussed in detail earlier in this guide.
:touch
If you set the :touch
option to true
, then the updated_at
or updated_on
timestamp on the associated object will be set to the current time whenever this object is saved or destroyed:
class Book < ApplicationRecord
belongs_to :, touch: true
end
class Author < ApplicationRecord
has_many :books
end
In this case, saving or destroying a book will update the timestamp on the associated author. You can also specify a particular timestamp attribute to update:
class Book < ApplicationRecord
belongs_to :, touch: :books_updated_at
end
:validate
If you set the :validate
option to true
, then associated objects will be validated whenever you save this object. By default, this is false
: associated objects will not be validated when this object is saved.
:optional
If you set the :optional
option to true
, then the presence of the associated
object won't be validated. By default, this option is set to false
.
Scopes for belongs_to
There may be times when you wish to customize the query used by belongs_to
. Such customizations can be achieved via a scope block. For example:
class Book < ApplicationRecord
belongs_to :, -> { where active: true }
end
You can use any of the standard querying methods inside the scope block. The following ones are discussed below:
where
includes
readonly
select
where
The where
method lets you specify the conditions that the associated object must meet.
class Book < ApplicationRecord
belongs_to :, -> { where active: true }
end
includes
You can use the includes
method to specify second-order associations that should be eager-loaded when this association is used. For example, consider these models:
class Chapter < ApplicationRecord
belongs_to :book
end
class Book < ApplicationRecord
belongs_to :
has_many :chapters
end
class Author < ApplicationRecord
has_many :books
end
If you frequently retrieve authors directly from chapters (@chapter.book.author
), then you can make your code somewhat more efficient by including authors in the association from chapters to books:
class Chapter < ApplicationRecord
belongs_to :book, -> { includes : }
end
class Book < ApplicationRecord
belongs_to :
has_many :chapters
end
class Author < ApplicationRecord
has_many :books
end
NOTE: There's no need to use includes
for immediate associations - that is, if you have Book belongs_to :author
, then the author is eager-loaded automatically when it's needed.
readonly
If you use readonly
, then the associated object will be read-only when retrieved via the association.
select
The select
method lets you override the SQL SELECT
clause that is used to retrieve data about the associated object. By default, Rails retrieves all columns.
TIP: If you use the select
method on a belongs_to
association, you should also set the :foreign_key
option to guarantee the correct results.
Do Any Associated Objects Exist?
You can see if any associated objects exist by using the association.nil?
method:
if @book. .nil?
@msg = "No author found for this book"
end
When are Objects Saved?
Assigning an object to a belongs_to
association does not automatically save the object. It does not save the associated object either.
has_one
Association Reference
The has_one
association creates a one-to-one match with another model. In database terms, this association says that the other class contains the foreign key. If this class contains the foreign key, then you should use belongs_to
instead.
Methods Added by has_one
When you declare a has_one
association, the declaring class automatically gains 6 methods related to the association:
association
association=(associate)
build_association(attributes = {})
create_association(attributes = {})
create_association!(attributes = {})
reload_association
In all of these methods, association
is replaced with the symbol passed as the first argument to has_one
. For example, given the declaration:
class Supplier < ApplicationRecord
has_one :account
end
Each instance of the Supplier
model will have these methods:
account
account=
build_account
create_account
create_account!
reload_account
NOTE: When initializing a new has_one
or belongs_to
association you must use the build_
prefix to build the association, rather than the association.build
method that would be used for has_many
or has_and_belongs_to_many
associations. To create one, use the create_
prefix.
association
The association
method returns the associated object, if any. If no associated object is found, it returns nil
.
@account = @supplier.account
If the associated object has already been retrieved from the database for this object, the cached version will be returned. To override this behavior (and force a database read), call #reload_association
on the parent object.
@account = @supplier.reload_account
association=(associate)
The association=
method assigns an associated object to this object. Behind the scenes, this means extracting the primary key from this object and setting the associated object's foreign key to the same value.
@supplier.account = @account
build_association(attributes = {})
The build_association
method returns a new object of the associated type. This object will be instantiated from the passed attributes, and the link through its foreign key will be set, but the associated object will not yet be saved.
@account = @supplier.build_account(terms: "Net 30")
create_association(attributes = {})
The create_association
method returns a new object of the associated type. This object will be instantiated from the passed attributes, the link through its foreign key will be set, and, once it passes all of the validations specified on the associated model, the associated object will be saved.
@account = @supplier.create_account(terms: "Net 30")
create_association!(attributes = {})
Does the same as create_association
above, but raises ::ActiveRecord::RecordInvalid
if the record is invalid.
Options for has_one
While Rails uses intelligent defaults that will work well in most situations, there may be times when you want to customize the behavior of the has_one
association reference. Such customizations can easily be accomplished by passing options when you create the association. For example, this association uses two such options:
class Supplier < ApplicationRecord
has_one :account, class_name: "Billing", dependent: :nullify
end
The has_one
association supports these options:
:as
:autosave
:class_name
:dependent
:foreign_key
:inverse_of
:primary_key
:source
:source_type
:through
:touch
:validate
:as
Setting the :as
option indicates that this is a polymorphic association. Polymorphic associations were discussed in detail earlier in this guide.
:autosave
If you set the :autosave
option to true
, Rails will save any loaded association members and destroy members that are marked for destruction whenever you save the parent object. Setting :autosave
to false
is not the same as not setting the :autosave
option. If the :autosave
option is not present, then new associated objects will be saved, but updated associated objects will not be saved.
:class_name
If the name of the other model cannot be derived from the association name, you can use the :class_name
option to supply the model name. For example, if a supplier has an account, but the actual name of the model containing accounts is Billing
, you'd set things up this way:
class Supplier < ApplicationRecord
has_one :account, class_name: "Billing"
end
:dependent
Controls what happens to the associated object when its owner is destroyed:
:destroy
causes the associated object to also be destroyed:delete
causes the associated object to be deleted directly from the database (so callbacks will not execute):destroy_async
: when the object is destroyed, an::ActiveRecord::DestroyAssociationAsyncJob
job is enqueued which will call destroy on its associated objects. Active Job must be set up for this to work.:nullify
causes the foreign key to be set toNULL
. Polymorphic type column is also nullified on polymorphic associations. Callbacks are not executed.:restrict_with_exception
causes an::ActiveRecord::DeleteRestrictionError
exception to be raised if there is an associated record:restrict_with_error
causes an error to be added to the owner if there is an associated object
It's necessary not to set or leave :nullify
option for those associations
that have NOT NULL
database constraints. If you don't set dependent
to
destroy such associations you won't be able to change the associated object
because the initial associated object's foreign key will be set to the
unallowed NULL
value.
:foreign_key
By convention, Rails assumes that the column used to hold the foreign key on the other model is the name of this model with the suffix _id
added. The :foreign_key
option lets you set the name of the foreign key directly:
class Supplier < ApplicationRecord
has_one :account, foreign_key: "supp_id"
end
TIP: In any case, Rails will not create foreign key columns for you. You need to explicitly define them as part of your migrations.
:inverse_of
The :inverse_of
option specifies the name of the belongs_to
association that is the inverse of this association.
class Supplier < ApplicationRecord
has_one :account, inverse_of: :supplier
end
class Account < ApplicationRecord
belongs_to :supplier, inverse_of: :account
end
:primary_key
By convention, Rails assumes that the column used to hold the primary key of this model is id
. You can override this and explicitly specify the primary key with the :primary_key
option.
:source
The :source
option specifies the source association name for a has_one :through
association.
:source_type
The :source_type
option specifies the source association type for a has_one :through
association that proceeds through a polymorphic association.
class Author < ApplicationRecord
has_one :book
has_one :hardback, through: :book, source: :format, source_type: "Hardback"
has_one :dust_jacket, through: :hardback
end
class Book < ApplicationRecord
belongs_to :format, polymorphic: true
end
class Paperback < ApplicationRecord; end
class Hardback < ApplicationRecord
has_one :dust_jacket
end
class DustJacket < ApplicationRecord; end
:through
The :through
option specifies a join model through which to perform the query. has_one :through
associations were discussed in detail earlier in this guide.
:touch
If you set the :touch
option to true
, then the updated_at
or updated_on
timestamp on the associated object will be set to the current time whenever this object is saved or destroyed:
class Supplier < ApplicationRecord
has_one :account, touch: true
end
class Account < ApplicationRecord
belongs_to :supplier
end
In this case, saving or destroying a supplier will update the timestamp on the associated account. You can also specify a particular timestamp attribute to update:
class Supplier < ApplicationRecord
has_one :account, touch: :suppliers_updated_at
end
:validate
If you set the :validate
option to true
, then associated objects will be validated whenever you save this object. By default, this is false
: associated objects will not be validated when this object is saved.
Scopes for has_one
There may be times when you wish to customize the query used by has_one
. Such customizations can be achieved via a scope block. For example:
class Supplier < ApplicationRecord
has_one :account, -> { where active: true }
end
You can use any of the standard querying methods inside the scope block. The following ones are discussed below:
where
includes
readonly
select
where
The where
method lets you specify the conditions that the associated object must meet.
class Supplier < ApplicationRecord
has_one :account, -> { where "confirmed = 1" }
end
includes
You can use the includes
method to specify second-order associations that should be eager-loaded when this association is used. For example, consider these models:
class Supplier < ApplicationRecord
has_one :account
end
class Account < ApplicationRecord
belongs_to :supplier
belongs_to :representative
end
class Representative < ApplicationRecord
has_many :accounts
end
If you frequently retrieve representatives directly from suppliers (@supplier.account.representative
), then you can make your code somewhat more efficient by including representatives in the association from suppliers to accounts:
class Supplier < ApplicationRecord
has_one :account, -> { includes :representative }
end
class Account < ApplicationRecord
belongs_to :supplier
belongs_to :representative
end
class Representative < ApplicationRecord
has_many :accounts
end
readonly
If you use the readonly
method, then the associated object will be read-only when retrieved via the association.
select
The select
method lets you override the SQL SELECT
clause that is used to retrieve data about the associated object. By default, Rails retrieves all columns.
Do Any Associated Objects Exist?
You can see if any associated objects exist by using the association.nil?
method:
if @supplier.account.nil?
@msg = "No account found for this supplier"
end
When are Objects Saved?
When you assign an object to a has_one
association, that object is automatically saved (in order to update its foreign key). In addition, any object being replaced is also automatically saved, because its foreign key will change too.
If either of these saves fails due to validation errors, then the assignment statement returns false
and the assignment itself is cancelled.
If the parent object (the one declaring the has_one
association) is unsaved (that is, new_record?
returns true
) then the child objects are not saved. They will automatically when the parent object is saved.
If you want to assign an object to a has_one
association without saving the object, use the build_association
method.
has_many
Association Reference
The has_many
association creates a one-to-many relationship with another model. In database terms, this association says that the other class will have a foreign key that refers to instances of this class.
Methods Added by has_many
When you declare a has_many
association, the declaring class automatically gains 17 methods related to the association:
collection
collection<<(object, ...)
collection.delete(object, ...)
collection.destroy(object, ...)
collection=(objects)
collection_singular_ids
collection_singular_ids=(ids)
collection.clear
collection.empty?
collection.size
collection.find(...)
collection.where(...)
collection.exists?(...)
collection.build(attributes = {})
collection.create(attributes = {})
collection.create!(attributes = {})
collection.reload
In all of these methods, collection
is replaced with the symbol passed as the first argument to has_many
, and collection_singular
is replaced with the singularized version of that symbol. For example, given the declaration:
class Author < ApplicationRecord
has_many :books
end
Each instance of the Author
model will have these methods:
books
books<<(object, ...)
books.delete(object, ...)
books.destroy(object, ...)
books=(objects)
book_ids
book_ids=(ids)
books.clear
books.empty?
books.size
books.find(...)
books.where(...)
books.exists?(...)
books.build(attributes = {}, ...)
books.create(attributes = {})
books.create!(attributes = {})
books.reload
collection
The collection
method returns a Relation of all of the associated objects. If there are no associated objects, it returns an empty Relation.
@books = @author.books
collection<<(object, ...)
The collection<<
method adds one or more objects to the collection by setting their foreign keys to the primary key of the calling model.
@author.books << @book1
collection.delete(object, ...)
The collection.delete
method removes one or more objects from the collection by setting their foreign keys to NULL
.
@author.books.delete(@book1)
WARNING: Additionally, objects will be destroyed if they're associated with dependent: :destroy
, and deleted if they're associated with dependent: :delete_all
.
collection.destroy(object, ...)
The collection.destroy
method removes one or more objects from the collection by running destroy
on each object.
@author.books.destroy(@book1)
WARNING: Objects will always be removed from the database, ignoring the :dependent
option.
collection=(objects)
The collection=
method makes the collection contain only the supplied objects, by adding and deleting as appropriate. The changes are persisted to the database.
collection_singular_ids
The collection_singular_ids
method returns an array of the ids of the objects in the collection.
@book_ids = @author.book_ids
collection_singular_ids=(ids)
The collection_singular_ids=
method makes the collection contain only the objects identified by the supplied primary key values, by adding and deleting as appropriate. The changes are persisted to the database.
collection.clear
The collection.clear
method removes all objects from the collection according to the strategy specified by the dependent
option. If no option is given, it follows the default strategy. The default strategy for has_many :through
associations is delete_all
, and for has_many
associations is to set the foreign keys to NULL
.
@author.books.clear
WARNING: Objects will be deleted if they're associated with dependent: :destroy
or dependent: :destroy_async
,
just like dependent: :delete_all
.
collection.empty?
The collection.empty?
method returns true
if the collection does not contain any associated objects.
<% if @author.books.empty? %>
No Books Found
<% end %>
collection.size
The collection.size
method returns the number of objects in the collection.
@book_count = @author.books.size
collection.find(...)
The collection.find
method finds objects within the collection's table.
@available_book = @author.books.find(1)
collection.where(...)
The collection.where
method finds objects within the collection based on the conditions supplied but the objects are loaded lazily meaning that the database is queried only when the object(s) are accessed.
@available_books = @author.books.where(available: true) # No query yet
@available_book = @available_books.first # Now the database will be queried
collection.exists?(...)
The collection.exists?
method checks whether an object meeting the supplied
conditions exists in the collection's table.
collection.build(attributes = {})
The collection.build
method returns a single or array of new objects of the associated type. The object(s) will be instantiated from the passed attributes, and the link through their foreign key will be created, but the associated objects will not yet be saved.
@book = @author.books.build(published_at: Time.now,
book_number: "A12345")
@books = @author.books.build([
{ published_at: Time.now, book_number: "A12346" },
{ published_at: Time.now, book_number: "A12347" }
])
collection.create(attributes = {})
The collection.create
method returns a single or array of new objects of the associated type. The object(s) will be instantiated from the passed attributes, the link through its foreign key will be created, and, once it passes all of the validations specified on the associated model, the associated object will be saved.
@book = @author.books.create(published_at: Time.now,
book_number: "A12345")
@books = @author.books.create([
{ published_at: Time.now, book_number: "A12346" },
{ published_at: Time.now, book_number: "A12347" }
])
collection.create!(attributes = {})
Does the same as collection.create
above, but raises ::ActiveRecord::RecordInvalid
if the record is invalid.
collection.reload
The collection.reload
method returns a Relation of all of the associated objects, forcing a database read. If there are no associated objects, it returns an empty Relation.
@books = @author.books.reload
Options for has_many
While Rails uses intelligent defaults that will work well in most situations, there may be times when you want to customize the behavior of the has_many
association reference. Such customizations can easily be accomplished by passing options when you create the association. For example, this association uses two such options:
class Author < ApplicationRecord
has_many :books, dependent: :delete_all, validate: false
end
The has_many
association supports these options:
:as
:autosave
:class_name
:counter_cache
:dependent
:foreign_key
:inverse_of
:primary_key
:source
:source_type
:through
:validate
:as
Setting the :as
option indicates that this is a polymorphic association, as discussed earlier in this guide.
:autosave
If you set the :autosave
option to true
, Rails will save any loaded association members and destroy members that are marked for destruction whenever you save the parent object. Setting :autosave
to false
is not the same as not setting the :autosave
option. If the :autosave
option is not present, then new associated objects will be saved, but updated associated objects will not be saved.
:class_name
If the name of the other model cannot be derived from the association name, you can use the :class_name
option to supply the model name. For example, if an author has many books, but the actual name of the model containing books is Transaction
, you'd set things up this way:
class Author < ApplicationRecord
has_many :books, class_name: "Transaction"
end
:counter_cache
This option can be used to configure a custom named :counter_cache
. You only need this option when you customized the name of your :counter_cache
on the belongs_to association.
:dependent
Controls what happens to the associated objects when their owner is destroyed:
:destroy
causes all the associated objects to also be destroyed:delete_all
causes all the associated objects to be deleted directly from the database (so callbacks will not execute):destroy_async
: when the object is destroyed, an::ActiveRecord::DestroyAssociationAsyncJob
job is enqueued which will call destroy on its associated objects. Active Job must be set up for this to work.:nullify
causes the foreign key to be set toNULL
. Polymorphic type column is also nullified on polymorphic associations. Callbacks are not executed.:restrict_with_exception
causes an::ActiveRecord::DeleteRestrictionError
exception to be raised if there are any associated records:restrict_with_error
causes an error to be added to the owner if there are any associated objects
The :destroy
and :delete_all
options also affect the semantics of the collection.delete
and collection=
methods by causing them to destroy associated objects when they are removed from the collection.
:foreign_key
By convention, Rails assumes that the column used to hold the foreign key on the other model is the name of this model with the suffix _id
added. The :foreign_key
option lets you set the name of the foreign key directly:
class Author < ApplicationRecord
has_many :books, foreign_key: "cust_id"
end
TIP: In any case, Rails will not create foreign key columns for you. You need to explicitly define them as part of your migrations.
:inverse_of
The :inverse_of
option specifies the name of the belongs_to
association that is the inverse of this association.
class Author < ApplicationRecord
has_many :books, inverse_of: :
end
class Book < ApplicationRecord
belongs_to :, inverse_of: :books
end
:primary_key
By convention, Rails assumes that the column used to hold the primary key of the association is id
. You can override this and explicitly specify the primary key with the :primary_key
option.
Let's say the users
table has id
as the primary_key but it also
has a guid
column. The requirement is that the todos
table should
hold the guid
column value as the foreign key and not id
value. This can be achieved like this:
class User < ApplicationRecord
has_many :todos, primary_key: :guid
end
Now if we execute @todo = @user.todos.create
then the @todo
record's user_id
value will be the guid
value of @user
.
:source
The :source
option specifies the source association name for a has_many :through
association. You only need to use this option if the name of the source association cannot be automatically inferred from the association name.
:source_type
The :source_type
option specifies the source association type for a has_many :through
association that proceeds through a polymorphic association.
class Author < ApplicationRecord
has_many :books
has_many :paperbacks, through: :books, source: :format, source_type: "Paperback"
end
class Book < ApplicationRecord
belongs_to :format, polymorphic: true
end
class Hardback < ApplicationRecord; end
class Paperback < ApplicationRecord; end
:through
The :through
option specifies a join model through which to perform the query. has_many :through
associations provide a way to implement many-to-many relationships, as discussed earlier in this guide.
:validate
If you set the :validate
option to false
, then associated objects will not be validated whenever you save this object. By default, this is true
: associated objects will be validated when this object is saved.
Scopes for has_many
There may be times when you wish to customize the query used by has_many
. Such customizations can be achieved via a scope block. For example:
class Author < ApplicationRecord
has_many :books, -> { where processed: true }
end
You can use any of the standard querying methods inside the scope block. The following ones are discussed below:
where
extending
group
includes
limit
offset
order
readonly
select
distinct
where
The where
method lets you specify the conditions that the associated object must meet.
class Author < ApplicationRecord
has_many :confirmed_books, -> { where "confirmed = 1" },
class_name: "Book"
end
You can also set conditions via a hash:
class Author < ApplicationRecord
has_many :confirmed_books, -> { where confirmed: true },
class_name: "Book"
end
If you use a hash-style where
option, then record creation via this association will be automatically scoped using the hash. In this case, using @author.confirmed_books.create
or @author.confirmed_books.build
will create books where the confirmed column has the value true
.
extending
The extending
method specifies a named module to extend the association proxy. Association extensions are discussed in detail later in this guide.
group
The group
method supplies an attribute name to group the result set by, using a GROUP BY
clause in the finder SQL.
class Author < ApplicationRecord
has_many :chapters, -> { group 'books.id' },
through: :books
end
includes
You can use the includes
method to specify second-order associations that should be eager-loaded when this association is used. For example, consider these models:
class Author < ApplicationRecord
has_many :books
end
class Book < ApplicationRecord
belongs_to :
has_many :chapters
end
class Chapter < ApplicationRecord
belongs_to :book
end
If you frequently retrieve chapters directly from authors (@author.books.chapters
), then you can make your code somewhat more efficient by including chapters in the association from authors to books:
class Author < ApplicationRecord
has_many :books, -> { includes :chapters }
end
class Book < ApplicationRecord
belongs_to :
has_many :chapters
end
class Chapter < ApplicationRecord
belongs_to :book
end
limit
The limit
method lets you restrict the total number of objects that will be fetched through an association.
class Author < ApplicationRecord
has_many :recent_books,
#=> { order('published_at desc').limit(100) },
class_name: "Book"
end
offset
The offset
method lets you specify the starting offset for fetching objects via an association. For example, -> { offset(11) }
will skip the first 11 records.
order
The order
method dictates the order in which associated objects will be received (in the syntax used by an SQL ORDER BY
clause).
class Author < ApplicationRecord
has_many :books, -> { order "date_confirmed DESC" }
end
readonly
If you use the readonly
method, then the associated objects will be read-only when retrieved via the association.
select
The select
method lets you override the SQL SELECT
clause that is used to retrieve data about the associated objects. By default, Rails retrieves all columns.
WARNING: If you specify your own select
, be sure to include the primary key and foreign key columns of the associated model. If you do not, Rails will throw an error.
distinct
Use the distinct
method to keep the collection free of duplicates. This is
mostly useful together with the :through
option.
class Person < ApplicationRecord
has_many :readings
has_many :articles, through: :readings
end
irb> person = Person.create(name: 'John')
irb> article = Article.create(name: 'a1')
irb> person.articles << article
irb> person.articles << article
irb> person.articles.to_a
=> [#<Article id: 5, name: "a1">, #<Article id: 5, name: "a1">]
irb> Reading.all.to_a
=> [#<Reading id: 12, person_id: 5, article_id: 5>, #<Reading id: 13, person_id: 5, article_id: 5>]
In the above case there are two readings and person.articles
brings out both of
them even though these records are pointing to the same article.
Now let's set distinct
:
class Person
has_many :readings
has_many :articles, -> { distinct }, through: :readings
end
irb> person = Person.create(name: 'Honda')
irb> article = Article.create(name: 'a1')
irb> person.articles << article
irb> person.articles << article
irb> person.articles.to_a
=> [#<Article id: 7, name: "a1">]
irb> Reading.all.to_a
=> [#<Reading id: 16, person_id: 7, article_id: 7>, #<Reading id: 17, person_id: 7, article_id: 7>]
In the above case there are still two readings. However person.articles
shows
only one article because the collection loads only unique records.
If you want to make sure that, upon insertion, all of the records in the
persisted association are distinct (so that you can be sure that when you
inspect the association that you will never find duplicate records), you should
add a unique index on the table itself. For example, if you have a table named
readings
and you want to make sure the articles can only be added to a person once,
you could add the following in a migration:
add_index :readings, [:person_id, :article_id], unique: true
Once you have this unique index, attempting to add the article to a person twice
will raise an ::ActiveRecord::RecordNotUnique
error:
irb> person = Person.create(name: 'Honda')
irb> article = Article.create(name: 'a1')
irb> person.articles << article
irb> person.articles << article
ActiveRecord::RecordNotUnique
Note that checking for uniqueness using something like include?
is subject
to race conditions. Do not attempt to use include?
to enforce distinctness
in an association. For instance, using the article example from above, the
following code would be racy because multiple users could be attempting this
at the same time:
person.articles << article unless person.articles.include?(article)
When are Objects Saved?
When you assign an object to a has_many
association, that object is automatically saved (in order to update its foreign key). If you assign multiple objects in one statement, then they are all saved.
If any of these saves fails due to validation errors, then the assignment statement returns false
and the assignment itself is cancelled.
If the parent object (the one declaring the has_many
association) is unsaved (that is, new_record?
returns true
) then the child objects are not saved when they are added. All unsaved members of the association will automatically be saved when the parent is saved.
If you want to assign an object to a has_many
association without saving the object, use the collection.build
method.
has_and_belongs_to_many
Association Reference
The has_and_belongs_to_many
association creates a many-to-many relationship with another model. In database terms, this associates two classes via an intermediate join table that includes foreign keys referring to each of the classes.
Methods Added by has_and_belongs_to_many
When you declare a has_and_belongs_to_many
association, the declaring class automatically gains several methods related to the association:
collection
collection<<(object, ...)
collection.delete(object, ...)
collection.destroy(object, ...)
collection=(objects)
collection_singular_ids
collection_singular_ids=(ids)
collection.clear
collection.empty?
collection.size
collection.find(...)
collection.where(...)
collection.exists?(...)
collection.build(attributes = {})
collection.create(attributes = {})
collection.create!(attributes = {})
collection.reload
In all of these methods, collection
is replaced with the symbol passed as the first argument to has_and_belongs_to_many
, and collection_singular
is replaced with the singularized version of that symbol. For example, given the declaration:
class Part < ApplicationRecord
has_and_belongs_to_many :assemblies
end
Each instance of the Part
model will have these methods:
assemblies
assemblies<<(object, ...)
assemblies.delete(object, ...)
assemblies.destroy(object, ...)
assemblies=(objects)
assembly_ids
assembly_ids=(ids)
assemblies.clear
assemblies.empty?
assemblies.size
assemblies.find(...)
assemblies.where(...)
assemblies.exists?(...)
assemblies.build(attributes = {}, ...)
assemblies.create(attributes = {})
assemblies.create!(attributes = {})
assemblies.reload
Additional Column Methods
If the join table for a has_and_belongs_to_many
association has additional columns beyond the two foreign keys, these columns will be added as attributes to records retrieved via that association. Records returned with additional attributes will always be read-only, because Rails cannot save changes to those attributes.
WARNING: The use of extra attributes on the join table in a has_and_belongs_to_many
association is deprecated. If you require this sort of complex behavior on the table that joins two models in a many-to-many relationship, you should use a has_many :through
association instead of has_and_belongs_to_many
.
collection
The collection
method returns a Relation of all of the associated objects. If there are no associated objects, it returns an empty Relation.
@assemblies = @part.assemblies
collection<<(object, ...)
The collection<<
method adds one or more objects to the collection by creating records in the join table.
@part.assemblies << @assembly1
NOTE: This method is aliased as collection.concat
and collection.push
.
collection.delete(object, ...)
The collection.delete
method removes one or more objects from the collection by deleting records in the join table. This does not destroy the objects.
@part.assemblies.delete(@assembly1)
collection.destroy(object, ...)
The collection.destroy
method removes one or more objects from the collection by deleting records in the join table. This does not destroy the objects.
@part.assemblies.destroy(@assembly1)
collection=(objects)
The collection=
method makes the collection contain only the supplied objects, by adding and deleting as appropriate. The changes are persisted to the database.
collection_singular_ids
The collection_singular_ids
method returns an array of the ids of the objects in the collection.
@assembly_ids = @part.assembly_ids
collection_singular_ids=(ids)
The collection_singular_ids=
method makes the collection contain only the objects identified by the supplied primary key values, by adding and deleting as appropriate. The changes are persisted to the database.
collection.clear
The collection.clear
method removes every object from the collection by deleting the rows from the joining table. This does not destroy the associated objects.
collection.empty?
The collection.empty?
method returns true
if the collection does not contain any associated objects.
<% if @part.assemblies.empty? %>
This part is not used in any assemblies
<% end %>
collection.size
The collection.size
method returns the number of objects in the collection.
@assembly_count = @part.assemblies.size
collection.find(...)
The collection.find
method finds objects within the collection's table.
@assembly = @part.assemblies.find(1)
collection.where(...)
The collection.where
method finds objects within the collection based on the conditions supplied but the objects are loaded lazily meaning that the database is queried only when the object(s) are accessed.
@new_assemblies = @part.assemblies.where("created_at > ?", 2.days.ago)
collection.exists?(...)
The collection.exists?
method checks whether an object meeting the supplied
conditions exists in the collection's table.
collection.build(attributes = {})
The collection.build
method returns a new object of the associated type. This object will be instantiated from the passed attributes, and the link through the join table will be created, but the associated object will not yet be saved.
@assembly = @part.assemblies.build({assembly_name: "Transmission housing"})
collection.create(attributes = {})
The collection.create
method returns a new object of the associated type. This object will be instantiated from the passed attributes, the link through the join table will be created, and, once it passes all of the validations specified on the associated model, the associated object will be saved.
@assembly = @part.assemblies.create({assembly_name: "Transmission housing"})
collection.create!(attributes = {})
Does the same as collection.create
, but raises ::ActiveRecord::RecordInvalid
if the record is invalid.
collection.reload
The collection.reload
method returns a Relation of all of the associated objects, forcing a database read. If there are no associated objects, it returns an empty Relation.
@assemblies = @part.assemblies.reload
Options for has_and_belongs_to_many
While Rails uses intelligent defaults that will work well in most situations, there may be times when you want to customize the behavior of the has_and_belongs_to_many
association reference. Such customizations can easily be accomplished by passing options when you create the association. For example, this association uses two such options:
class Parts < ApplicationRecord
has_and_belongs_to_many :assemblies, -> { readonly },
autosave: true
end
The has_and_belongs_to_many
association supports these options:
:association_foreign_key
:autosave
:class_name
:foreign_key
:join_table
:validate
:association_foreign_key
By convention, Rails assumes that the column in the join table used to hold the foreign key pointing to the other model is the name of that model with the suffix _id
added. The :association_foreign_key
option lets you set the name of the foreign key directly:
TIP: The :foreign_key
and :association_foreign_key
options are useful when setting up a many-to-many self-join. For example:
class User < ApplicationRecord
has_and_belongs_to_many :friends,
class_name: "User",
foreign_key: "this_user_id",
association_foreign_key: "other_user_id"
end
:autosave
If you set the :autosave
option to true
, Rails will save any loaded association members and destroy members that are marked for destruction whenever you save the parent object. Setting :autosave
to false
is not the same as not setting the :autosave
option. If the :autosave
option is not present, then new associated objects will be saved, but updated associated objects will not be saved.
:class_name
If the name of the other model cannot be derived from the association name, you can use the :class_name
option to supply the model name. For example, if a part has many assemblies, but the actual name of the model containing assemblies is Gadget
, you'd set things up this way:
class Parts < ApplicationRecord
has_and_belongs_to_many :assemblies, class_name: "Gadget"
end
:foreign_key
By convention, Rails assumes that the column in the join table used to hold the foreign key pointing to this model is the name of this model with the suffix _id
added. The :foreign_key
option lets you set the name of the foreign key directly:
class User < ApplicationRecord
has_and_belongs_to_many :friends,
class_name: "User",
foreign_key: "this_user_id",
association_foreign_key: "other_user_id"
end
:join_table
If the default name of the join table, based on lexical ordering, is not what you want, you can use the :join_table
option to override the default.
:validate
If you set the :validate
option to false
, then associated objects will not be validated whenever you save this object. By default, this is true
: associated objects will be validated when this object is saved.
Scopes for has_and_belongs_to_many
There may be times when you wish to customize the query used by has_and_belongs_to_many
. Such customizations can be achieved via a scope block. For example:
class Parts < ApplicationRecord
has_and_belongs_to_many :assemblies, -> { where active: true }
end
You can use any of the standard querying methods inside the scope block. The following ones are discussed below:
where
extending
group
includes
limit
offset
order
readonly
select
distinct
where
The where
method lets you specify the conditions that the associated object must meet.
class Parts < ApplicationRecord
has_and_belongs_to_many :assemblies,
#=> { where "factory = 'Seattle'" }
end
You can also set conditions via a hash:
class Parts < ApplicationRecord
has_and_belongs_to_many :assemblies,
#=> { where factory: 'Seattle' }
end
If you use a hash-style where
, then record creation via this association will be automatically scoped using the hash. In this case, using @parts.assemblies.create
or @parts.assemblies.build
will create orders where the factory
column has the value "Seattle".
extending
The extending
method specifies a named module to extend the association proxy. Association extensions are discussed in detail later in this guide.
group
The group
method supplies an attribute name to group the result set by, using a GROUP BY
clause in the finder SQL.
class Parts < ApplicationRecord
has_and_belongs_to_many :assemblies, -> { group "factory" }
end
includes
You can use the includes
method to specify second-order associations that should be eager-loaded when this association is used.
limit
The limit
method lets you restrict the total number of objects that will be fetched through an association.
class Parts < ApplicationRecord
has_and_belongs_to_many :assemblies,
#=> { order("created_at DESC").limit(50) }
end
offset
The offset
method lets you specify the starting offset for fetching objects via an association. For example, if you set offset(11)
, it will skip the first 11 records.
order
The order
method dictates the order in which associated objects will be received (in the syntax used by an SQL ORDER BY
clause).
class Parts < ApplicationRecord
has_and_belongs_to_many :assemblies,
#=> { order "assembly_name ASC" }
end
readonly
If you use the readonly
method, then the associated objects will be read-only when retrieved via the association.
select
The select
method lets you override the SQL SELECT
clause that is used to retrieve data about the associated objects. By default, Rails retrieves all columns.
distinct
Use the distinct
method to remove duplicates from the collection.
When are Objects Saved?
When you assign an object to a has_and_belongs_to_many
association, that object is automatically saved (in order to update the join table). If you assign multiple objects in one statement, then they are all saved.
If any of these saves fails due to validation errors, then the assignment statement returns false
and the assignment itself is cancelled.
If the parent object (the one declaring the has_and_belongs_to_many
association) is unsaved (that is, new_record?
returns true
) then the child objects are not saved when they are added. All unsaved members of the association will automatically be saved when the parent is saved.
If you want to assign an object to a has_and_belongs_to_many
association without saving the object, use the collection.build
method.
Association Callbacks
Normal callbacks hook into the life cycle of Active Record objects, allowing you to work with those objects at various points. For example, you can use a :before_save
callback to cause something to happen just before an object is saved.
Association callbacks are similar to normal callbacks, but they are triggered by events in the life cycle of a collection. There are four available association callbacks:
before_add
after_add
before_remove
after_remove
You define association callbacks by adding options to the association declaration. For example:
class Author < ApplicationRecord
has_many :books, before_add: :check_credit_limit
def check_credit_limit(book)
# ...
end
end
Rails passes the object being added or removed to the callback.
You can stack callbacks on a single event by passing them as an array:
class Author < ApplicationRecord
has_many :books,
before_add: [:check_credit_limit, :calculate_shipping_charges]
def check_credit_limit(book)
# ...
end
def calculate_shipping_charges(book)
# ...
end
end
If a before_add
callback throws :abort
, the object does not get added to
the collection. Similarly, if a before_remove
callback throws :abort
, the
object does not get removed from the collection:
# book won't be added if the limit has been reached
def check_credit_limit(book)
throw(:abort) if limit_reached?
end
NOTE: These callbacks are called only when the associated objects are added or removed through the association collection:
# Triggers `before_add` callback
.books << book
.books = [book, book2]
# Does not trigger the `before_add` callback
book.update(author_id: 1)
Association Extensions
You're not limited to the functionality that Rails automatically builds into association proxy objects. You can also extend these objects through anonymous modules, adding new finders, creators, or other methods. For example:
class Author < ApplicationRecord
has_many :books do
def find_by_book_prefix(book_number)
find_by(category_id: book_number[0..2])
end
end
end
If you have an extension that should be shared by many associations, you can use a named extension module. For example:
module FindRecentExtension
def find_recent
where("created_at > ?", 5.days.ago)
end
end
class Author < ApplicationRecord
has_many :books, -> { extending FindRecentExtension }
end
class Supplier < ApplicationRecord
has_many :deliveries, -> { extending FindRecentExtension }
end
Extensions can refer to the internals of the association proxy using these three attributes of the proxy_association
accessor:
proxy_association.owner
returns the object that the association is a part of.proxy_association.reflection
returns the reflection object that describes the association.proxy_association.target
returns the associated object forbelongs_to
orhas_one
, or the collection of associated objects forhas_many
orhas_and_belongs_to_many
.
Single Table Inheritance (STI)
Sometimes, you may want to share fields and behavior between different models.
Let's say we have Car, Motorcycle, and Bicycle models. We will want to share
the color
and price
fields and some methods for all of them, but having some
specific behavior for each, and separated controllers too.
First, let's generate the base Vehicle model:
$ bin/rails generate model vehicle type:string color:string price:decimal{10.2}
Did you note we are adding a "type" field? Since all models will be saved in a single database table, Rails will save in this column the name of the model that is being saved. In our example, this can be "Car", "Motorcycle" or "Bicycle." STI won't work without a "type" field in the table.
Next, we will generate the Car model that inherits from Vehicle. For this,
we can use the --parent=PARENT
option, which will generate a model that
inherits from the specified parent and without equivalent migration (since the
table already exists).
For example, to generate the Car model:
$ bin/rails generate model car --parent=Vehicle
The generated model will look like this:
class Car < Vehicle
end
This means that all behavior added to Vehicle is available for Car too, as associations, public methods, etc.
Creating a car will save it in the vehicles
table with "Car" as the type
field:
Car.create(color: 'Red', price: 10000)
will generate the following SQL:
INSERT INTO "vehicles" ("type", "color", "price") VALUES ('Car', 'Red', 10000)
Querying car records will search only for vehicles that are cars:
Car.all
will run a query like:
SELECT "vehicles".* FROM "vehicles" WHERE "vehicles"."type" IN ('Car')


Video
ABOUT SCARF
SCARF is a young foundation recently founded by Steve Gijrath and Ellie Percey in aid of refugees of war in the Middle East.
Our aim is multi-faceted. First, we endeavour to contribute to the volunteer effort to collect, supply and distribute clothing and aid through the support and donations and individuals both in the local community and internationally. Second, we are contributing to a volunteer/foundational effort to approach the currently diffuse network of volunteers and groups aimed at education and skills development of refugee children, and create a united network of skilled volunteers willing to share their creative and educative expertise with the young children who are missing out on such key stages of development. Beginning with experimental workshops (creative, entertaining and educative) at various camps on the Greek islands, we hop to connect with other networks doing the same thing, and unite.
OUR STORY
At the end of 2015, co-founders Steve and Ellie met while working as volunteers on Leros, a small Greek island near Turkey. At the time their work relied hugely on, and benefitted greatly from, the support of generous donations on their individual fundraising pages, as did refugees of war for whom warm clothes, shoes and baby bottles were bought; so necessary for their continuing journey.
At the Leros camp, children were entertained by clowns, bubbles, drawing, painting and music while they waited for the next stage of their journey.
Since returning from Leros Steve and Ellie have visited Syrian friends who were quick to pull out their walking boots, supplied on Leros, from under their beds, as a reminder of their gratitude. One thing that is constantly astounding about the majority of refugees we met on Leros is the level of their graciousness and gratefulness when receiving aid - the help given to them is something they will never forget.
This was the inspiration for SCARF.