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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?
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 customers and a model for orders. Each customer can have many orders. Without associations, the model declarations would look like this:
class Customer < ActiveRecord::Base
end
class Order < ActiveRecord::Base
end
Now, suppose we wanted to add a new order for an existing customer. We'd need to do something like this:
@order = Order.create(order_date: Time.now, customer_id: @customer.id)
Or consider deleting a customer, and ensuring that all of its orders get deleted as well:
@orders = Order.where(customer_id: @customer.id)
@orders.each do |order|
order.destroy
end
@customer.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 customers and orders:
class Customer < ActiveRecord::Base
has_many :orders, dependent: :destroy
end
class Order < ActiveRecord::Base
belongs_to :customer
end
With this change, creating a new order for a particular customer is easier:
@order = @customer.orders.create(order_date: Time.now)
Deleting a customer and all of its orders is much easier:
@customer.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
In Rails, an association is a connection between two Active Record models. 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. Rails supports six types of associations:
belongs_to
has_one
has_many
has_many :through
has_one :through
has_and_belongs_to_many
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 one-to-one 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 customers and orders, and each order can be assigned to exactly one customer, you'd declare the order model this way:
class Order < ActiveRecord::Base
belongs_to :customer
end
NOTE: belongs_to
associations must use the singular term. If you used the pluralized form in the above example for the customer
association in the Order
model, you would be told that there was an "uninitialized constant Order::Customers". 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 CreateOrders < ActiveRecord::Migration
def change
create_table :customers do |t|
t.string :name
t.timestamps
end
create_table :orders do |t|
t.belongs_to :customer
t.datetime :order_date
t.timestamps
end
end
end
The has_one
Association
A has_one
association also sets up a one-to-one connection with another model, but with somewhat different semantics (and consequences). This association indicates that each instance of a model contains or possesses one instance of another model. For example, if each supplier in your application has only one account, you'd declare the supplier model like this:
class Supplier < ActiveRecord::Base
has_one :account
end
The corresponding migration might look like this:
class CreateSuppliers < ActiveRecord::Migration
def change
create_table :suppliers do |t|
t.string :name
t.timestamps
end
create_table :accounts do |t|
t.belongs_to :supplier
t.string :account_number
t.timestamps
end
end
end
The has_many
Association
A has_many
association 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 customers and orders, the customer model could be declared like this:
class Customer < ActiveRecord::Base
has_many :orders
end
NOTE: The name of the other model is pluralized when declaring a has_many
association.
The corresponding migration might look like this:
class CreateCustomers < ActiveRecord::Migration
def change
create_table :customers do |t|
t.string :name
t.timestamps
end
create_table :orders do |t|
t.belongs_to :customer
t.datetime :order_date
t.timestamps
end
end
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 < ActiveRecord::Base
has_many :appointments
has_many :patients, through: :appointments
end
class Appointment < ActiveRecord::Base
belongs_to :physician
belongs_to :patient
end
class Patient < ActiveRecord::Base
has_many :appointments
has_many :physicians, through: :appointments
end
The corresponding migration might look like this:
class CreateAppointments < ActiveRecord::Migration
def change
create_table :physicians do |t|
t.string :name
t.timestamps
end
create_table :patients do |t|
t.string :name
t.timestamps
end
create_table :appointments do |t|
t.belongs_to :physician
t.belongs_to :patient
t.datetime :appointment_date
t.timestamps
end
end
end
The collection of join models can be managed via the API. For example, if you assign
physician.patients = patients
new join models are created for newly associated objects, and if some are gone their rows are 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 < ActiveRecord::Base
has_many :sections
has_many :paragraphs, through: :sections
end
class Section < ActiveRecord::Base
belongs_to :document
has_many :paragraphs
end
class Paragraph < ActiveRecord::Base
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 < ActiveRecord::Base
has_one :account
has_one :account_history, through: :account
end
class Account < ActiveRecord::Base
belongs_to :supplier
has_one :account_history
end
class AccountHistory < ActiveRecord::Base
belongs_to :account
end
The corresponding migration might look like this:
class CreateAccountHistories < ActiveRecord::Migration
def change
create_table :suppliers do |t|
t.string :name
t.timestamps
end
create_table :accounts do |t|
t.belongs_to :supplier
t.string :account_number
t.timestamps
end
create_table :account_histories do |t|
t.belongs_to :account
t.integer :credit_rating
t.timestamps
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. 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 < ActiveRecord::Base
has_and_belongs_to_many :parts
end
class Part < ActiveRecord::Base
has_and_belongs_to_many :assemblies
end
The corresponding migration might look like this:
class CreateAssembliesAndParts < ActiveRecord::Migration
def change
create_table :assemblies do |t|
t.string :name
t.timestamps
end
create_table :parts do |t|
t.string :part_number
t.timestamps
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 < ActiveRecord::Base
has_one :account
end
class Account < ActiveRecord::Base
belongs_to :supplier
end
The corresponding migration might look like this:
class CreateSuppliers < ActiveRecord::Migration
def change
create_table :suppliers do |t|
t.string :name
t.timestamps
end
create_table :accounts do |t|
t.integer :supplier_id
t.string :account_number
t.timestamps
end
end
end
NOTE: Using t.integer :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 simpler way is to use has_and_belongs_to_many
, which allows you to make the association directly:
class Assembly < ActiveRecord::Base
has_and_belongs_to_many :parts
end
class Part < ActiveRecord::Base
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 < ActiveRecord::Base
has_many :manifests
has_many :parts, through: :manifests
end
class Manifest < ActiveRecord::Base
belongs_to :assembly
belongs_to :part
end
class Part < ActiveRecord::Base
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 < ActiveRecord::Base
belongs_to :imageable, polymorphic: true
end
class Employee < ActiveRecord::Base
has_many :pictures, as: :imageable
end
class Product < ActiveRecord::Base
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
def change
create_table :pictures do |t|
t.string :name
t.integer :imageable_id
t.string :imageable_type
t.timestamps
end
end
end
This migration can be simplified by using the t.references
form:
class CreatePictures < ActiveRecord::Migration
def change
create_table :pictures do |t|
t.string :name
t.references :imageable, polymorphic: true
t.timestamps
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 < ActiveRecord::Base
has_many :subordinates, class_name: "Employee",
foreign_key: "manager_id"
belongs_to :manager, class_name: "Employee"
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
def change
create_table :employees do |t|
t.references :manager
t.timestamps
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:
customer.orders # retrieves orders from the database
customer.orders.size # uses the cached copy of orders
customer.orders.empty? # uses the cached copy of orders
But what if you want to reload the cache, because data might have been changed by some other part of the application? Just pass true
to the association call:
customer.orders # retrieves orders from the database
customer.orders.size # uses the cached copy of orders
customer.orders(true).empty? # discards the cached copy of orders
# and goes back to the database
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 Order < ActiveRecord::Base
belongs_to :customer
end
This declaration needs to be backed up by the proper foreign key declaration on the orders table:
class CreateOrders < ActiveRecord::Migration
def change
create_table :orders do |t|
t.datetime :order_date
t.string :order_number
t.integer :customer_id
end
end
end
If you create an association some time after you build the underlying model, you need to remember to create an add_column
migration to provide the necessary foreign key.
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 customer and order models will give the default join table name of "customers_orders" because "c" outranks "o" 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 < ActiveRecord::Base
has_and_belongs_to_many :parts
end
class Part < ActiveRecord::Base
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
def change
create_table :assemblies_parts, id: false do |t|
t.integer :assembly_id
t.integer :part_id
end
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 models IDs, or exceptions about conflicting IDs, chances are you forgot that bit.
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 < ActiveRecord::Base
has_one :account
end
class Account < ActiveRecord::Base
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 < ActiveRecord::Base
has_one :account
end
end
module Billing
class Account < ActiveRecord::Base
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 < ActiveRecord::Base
has_one :account,
class_name: "MyApplication::Billing::Account"
end
end
module Billing
class Account < ActiveRecord::Base
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 Customer < ActiveRecord::Base
has_many :orders
end
class Order < ActiveRecord::Base
belongs_to :customer
end
By default, Active Record doesn't know about the connection between these associations. This can lead to two copies of an object getting out of sync:
c = Customer.first
o = c.orders.first
c.first_name == o.customer.first_name # => true
c.first_name = 'Manny'
c.first_name == o.customer.first_name # => false
This happens because c and o.customer are two different in-memory representations of the same data, and neither one is automatically refreshed from changes to the other. Active Record provides the :inverse_of
option so that you can inform it of these relations:
class Customer < ActiveRecord::Base
has_many :orders, inverse_of: :customer
end
class Order < ActiveRecord::Base
belongs_to :customer, inverse_of: :orders
end
With these changes, Active Record will only load one copy of the customer object, preventing inconsistencies and making your application more efficient:
c = Customer.first
o = c.orders.first
c.first_name == o.customer.first_name # => true
c.first_name = 'Manny'
c.first_name == o.customer.first_name # => true
There are a few limitations to inverse_of
support:
- They do not work with
:through
associations. - They do not work with
:polymorphic
associations. - They do not work with
:as
associations. - For
belongs_to
associations,has_many
inverse associations are ignored.
Every association will attempt to automatically find the inverse association
and set the :inverse_of
option heuristically (based on the association name).
Most associations with standard names will be supported. However, associations
that contain the following options will not have their inverses set
automatically:
- :conditions
- :through
- :polymorphic
- :foreign_key
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
The belongs_to
association creates a one-to-one match with another model. In database terms, this association says that this class contains the foreign key. If the other class contains the foreign key, then you should use has_one
instead.
Methods Added by belongs_to
When you declare a belongs_to
association, the declaring class automatically gains five methods related to the association:
association(force_reload = false)
association=(associate)
build_association(attributes = {})
create_association(attributes = {})
create_association!(attributes = {})
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 Order < ActiveRecord::Base
belongs_to :customer
end
Each instance of the order model will have these methods:
customer
customer=
build_customer
create_customer
create_customer!
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(force_reload = false)
The association
method returns the associated object, if any. If no associated object is found, it returns nil
.
@customer = @order.customer
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), pass true
as the force_reload
argument.
association=(associate)
The association=
method assigns an associated object to this object. Behind the scenes, this means extracting the primary key from the associate object and setting this object's foreign key to the same value.
@order.customer = @customer
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.
@customer = @order.build_customer(customer_number: 123,
customer_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.
@customer = @order.create_customer(customer_number: 123,
customer_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 Order < ActiveRecord::Base
belongs_to :customer, dependent: :destroy,
counter_cache: true
end
The belongs_to
association supports these options:
:autosave
:class_name
:counter_cache
:dependent
:foreign_key
:inverse_of
:polymorphic
:touch
:validate
:autosave
If you set the :autosave
option to true
, Rails will save any loaded members and destroy members that are marked for destruction whenever you save the parent object.
: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 order belongs to a customer, but the actual name of the model containing customers is Patron
, you'd set things up this way:
class Order < ActiveRecord::Base
belongs_to :customer, 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 Order < ActiveRecord::Base
belongs_to :customer
end
class Customer < ActiveRecord::Base
has_many :orders
end
With these declarations, asking for the value of @customer.orders.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 Order < ActiveRecord::Base
belongs_to :customer, counter_cache: true
end
class Customer < ActiveRecord::Base
has_many :orders
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 model. In the case above, you would need to add a column named orders_count
to the Customer
model. You can override the default column name if you need to:
class Order < ActiveRecord::Base
belongs_to :customer, counter_cache: :count_of_orders
end
class Customer < ActiveRecord::Base
has_many :orders
end
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.
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 Order < ActiveRecord::Base
belongs_to :customer, 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.
:inverse_of
The :inverse_of
option specifies the name of the has_many
or has_one
association that is the inverse of this association. Does not work in combination with the :polymorphic
options.
class Customer < ActiveRecord::Base
has_many :orders, inverse_of: :customer
end
class Order < ActiveRecord::Base
belongs_to :customer, inverse_of: :orders
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 Order < ActiveRecord::Base
belongs_to :customer, touch: true
end
class Customer < ActiveRecord::Base
has_many :orders
end
In this case, saving or destroying an order will update the timestamp on the associated customer. You can also specify a particular timestamp attribute to update:
class Order < ActiveRecord::Base
belongs_to :customer, touch: :orders_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 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 Order < ActiveRecord::Base
belongs_to :customer, -> { where active: true },
dependent: :destroy
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 Order < ActiveRecord::Base
belongs_to :customer, -> { 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 LineItem < ActiveRecord::Base
belongs_to :order
end
class Order < ActiveRecord::Base
belongs_to :customer
has_many :line_items
end
class Customer < ActiveRecord::Base
has_many :orders
end
If you frequently retrieve customers directly from line items (@line_item.order.customer
), then you can make your code somewhat more efficient by including customers in the association from line items to orders:
class LineItem < ActiveRecord::Base
belongs_to :order, -> { includes :customer }
end
class Order < ActiveRecord::Base
belongs_to :customer
has_many :line_items
end
class Customer < ActiveRecord::Base
has_many :orders
end
NOTE: There's no need to use includes
for immediate associations - that is, if you have Order belongs_to :customer
, then the customer 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 @order.customer.nil?
@msg = "No customer found for this order"
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 five methods related to the association:
association(force_reload = false)
association=(associate)
build_association(attributes = {})
create_association(attributes = {})
create_association!(attributes = {})
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 < ActiveRecord::Base
has_one :account
end
Each instance of the Supplier
model will have these methods:
account
account=
build_account
create_account
create_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(force_reload = false)
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), pass true
as the force_reload
argument.
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 associate 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 < ActiveRecord::Base
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
: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 members and destroy members that are marked for destruction whenever you save the parent object.
: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 < ActiveRecord::Base
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):nullify
causes the foreign key to be set toNULL
. Callbacks are not executed.:restrict_with_exception
causes an 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 initial associated object foreign key will be set to 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 < ActiveRecord::Base
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. Does not work in combination with the :through
or :as
options.
class Supplier < ActiveRecord::Base
has_one :account, inverse_of: :supplier
end
class Account < ActiveRecord::Base
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.
: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.
: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 < ActiveRecord::Base
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 < ActiveRecord::Base
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 < ActiveRecord::Base
has_one :account
end
class Account < ActiveRecord::Base
belongs_to :supplier
belongs_to :representative
end
class Representative < ActiveRecord::Base
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 < ActiveRecord::Base
has_one :account, -> { includes :representative }
end
class Account < ActiveRecord::Base
belongs_to :supplier
belongs_to :representative
end
class Representative < ActiveRecord::Base
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 association.build
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 16 methods related to the association:
collection(force_reload = false)
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 = {})
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 Customer < ActiveRecord::Base
has_many :orders
end
Each instance of the customer model will have these methods:
orders(force_reload = false)
orders<<(object, ...)
orders.delete(object, ...)
orders.destroy(object, ...)
orders=objects
order_ids
order_ids=ids
orders.clear
orders.empty?
orders.size
orders.find(...)
orders.where(...)
orders.exists?(...)
orders.build(attributes = {}, ...)
orders.create(attributes = {})
orders.create!(attributes = {})
collection(force_reload = false)
The collection
method returns an array of all of the associated objects. If there are no associated objects, it returns an empty array.
@orders = @customer.orders
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.
@customer.orders << @order1
collection.delete(object, ...)
The collection.delete
method removes one or more objects from the collection by setting their foreign keys to NULL
.
@customer.orders.delete(@order1)
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.
@customer.orders.destroy(@order1)
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.
collection_singular_ids
The collection_singular_ids
method returns an array of the ids of the objects in the collection.
@order_ids = @customer.order_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.
collection.clear
The collection.clear
method removes every object from the collection. This destroys the associated objects if they are associated with dependent: :destroy
, deletes them directly from the database if dependent: :delete_all
, and otherwise sets their foreign keys to NULL
.
collection.empty?
The collection.empty?
method returns true
if the collection does not contain any associated objects.
<% if @customer.orders.empty? %>
No Orders Found
<% end %>
collection.size
The collection.size
method returns the number of objects in the collection.
@order_count = @customer.orders.size
collection.find(...)
The collection.find
method finds objects within the collection. It uses the same syntax and options as ActiveRecord::Base.find
.
@open_orders = @customer.orders.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.
@open_orders = @customer.orders.where(open: true) # No query yet
@open_order = @open_orders.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. It uses the same syntax and options as ActiveRecord::Base.exists?
.
collection.build(attributes = {}, ...)
The collection.build
method returns one or more new objects of the associated type. These objects 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.
@order = @customer.orders.build(order_date: Time.now,
order_number: "A12345")
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 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.
@order = @customer.orders.create(order_date: Time.now,
order_number: "A12345")
collection.create!(attributes = {})
Does the same as collection.create
above, but raises ActiveRecord::RecordInvalid
if the record is invalid.
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 Customer < ActiveRecord::Base
has_many :orders, dependent: :delete_all, validate: :false
end
The has_many
association supports these options:
:as
:autosave
:class_name
: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 members and destroy members that are marked for destruction whenever you save the parent object.
: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 customer has many orders, but the actual name of the model containing orders is Transaction
, you'd set things up this way:
class Customer < ActiveRecord::Base
has_many :orders, class_name: "Transaction"
end
: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):nullify
causes the foreign keys to be set toNULL
. Callbacks are not executed.:restrict_with_exception
causes an 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
NOTE: This option is ignored when you use the :through
option on the association.
: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 Customer < ActiveRecord::Base
has_many :orders, 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. Does not work in combination with the :through
or :as
options.
class Customer < ActiveRecord::Base
has_many :orders, inverse_of: :customer
end
class Order < ActiveRecord::Base
belongs_to :customer, inverse_of: :orders
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 that users
table has id
as the primary_key but it also has
guid
column. And the requirement is that todos
table should hold
guid
column value and not id
value. This can be achieved like this
class User < ActiveRecord::Base
has_many :todos, primary_key: :guid
end
Now if we execute @user.todos.create
then @todo
record will have
user_id
value as 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.
: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 Customer < ActiveRecord::Base
has_many :orders, -> { 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
uniq
where
The where
method lets you specify the conditions that the associated object must meet.
class Customer < ActiveRecord::Base
has_many :confirmed_orders, -> { where "confirmed = 1" },
class_name: "Order"
end
You can also set conditions via a hash:
class Customer < ActiveRecord::Base
has_many :confirmed_orders, -> { where confirmed: true },
class_name: "Order"
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 @customer.confirmed_orders.create
or @customer.confirmed_orders.build
will create orders 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 Customer < ActiveRecord::Base
has_many :line_items, -> { group 'orders.id' },
through: :orders
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 Customer < ActiveRecord::Base
has_many :orders
end
class Order < ActiveRecord::Base
belongs_to :customer
has_many :line_items
end
class LineItem < ActiveRecord::Base
belongs_to :order
end
If you frequently retrieve line items directly from customers (@customer.orders.line_items
), then you can make your code somewhat more efficient by including line items in the association from customers to orders:
class Customer < ActiveRecord::Base
has_many :orders, -> { includes :line_items }
end
class Order < ActiveRecord::Base
belongs_to :customer
has_many :line_items
end
class LineItem < ActiveRecord::Base
belongs_to :order
end
limit
The limit
method lets you restrict the total number of objects that will be fetched through an association.
class Customer < ActiveRecord::Base
has_many :recent_orders,
-> { order('order_date desc').limit(100) },
class_name: "Order",
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 Customer < ActiveRecord::Base
has_many :orders, -> { 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 < ActiveRecord::Base
has_many :readings
has_many :posts, through: :readings
end
person = Person.create(name: 'John')
post = Post.create(name: 'a1')
person.posts << post
person.posts << post
person.posts.inspect # => [#<Post id: 5, name: "a1">, #<Post id: 5, name: "a1">]
Reading.all.inspect # => [#<Reading id: 12, person_id: 5, post_id: 5>, #<Reading id: 13, person_id: 5, post_id: 5>]
In the above case there are two readings and person.posts
brings out both of
them even though these records are pointing to the same post.
Now let's set distinct
:
class Person
has_many :readings
has_many :posts, -> { distinct }, through: :readings
end
person = Person.create(name: 'Honda')
post = Post.create(name: 'a1')
person.posts << post
person.posts << post
person.posts.inspect # => [#<Post id: 7, name: "a1">]
Reading.all.inspect # => [#<Reading id: 16, person_id: 7, post_id: 7>, #<Reading id: 17, person_id: 7, post_id: 7>]
In the above case there are still two readings. However person.posts
shows
only one post 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
person_posts
and you want to make sure all the posts are unique, you could
add the following in a migration:
add_index :person_posts, :post, unique: true
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 post example from above, the
following code would be racy because multiple users could be attempting this
at the same time:
person.posts << post unless person.posts.include?(post)
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 16 methods related to the association:
collection(force_reload = false)
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 = {})
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 < ActiveRecord::Base
has_and_belongs_to_many :assemblies
end
Each instance of the part model will have these methods:
assemblies(force_reload = false)
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 = {})
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(force_reload = false)
The collection
method returns an array of all of the associated objects. If there are no associated objects, it returns an empty array.
@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)
WARNING: This does not trigger callbacks on the join records.
collection.destroy(object, ...)
The collection.destroy
method removes one or more objects from the collection by running destroy
on each record in the join table, including running callbacks. 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.
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.
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. It uses the same syntax and options as ActiveRecord::Base.find
. It also adds the additional condition that the object must be in the collection.
@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. It also adds the additional condition that the object must be in the collection.
@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. It uses the same syntax and options as ActiveRecord::Base.exists?
.
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.
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 < ActiveRecord::Base
has_and_belongs_to_many :assemblies, autosave: true,
readonly: true
end
The has_and_belongs_to_many
association supports these options:
:association_foreign_key
:autosave
:class_name
:foreign_key
:join_table
:validate
:readonly
: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 < ActiveRecord::Base
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 members and destroy members that are marked for destruction whenever you save the parent object.
: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 < ActiveRecord::Base
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 < ActiveRecord::Base
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 < ActiveRecord::Base
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
uniq
where
The where
method lets you specify the conditions that the associated object must meet.
class Parts < ActiveRecord::Base
has_and_belongs_to_many :assemblies,
-> { where "factory = 'Seattle'" }
end
You can also set conditions via a hash:
class Parts < ActiveRecord::Base
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 < ActiveRecord::Base
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 < ActiveRecord::Base
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 < ActiveRecord::Base
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.
uniq
Use the uniq
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 Customer < ActiveRecord::Base
has_many :orders, before_add: :check_credit_limit
def check_credit_limit(order)
...
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 Customer < ActiveRecord::Base
has_many :orders,
before_add: [:check_credit_limit, :calculate_shipping_charges]
def check_credit_limit(order)
...
end
def calculate_shipping_charges(order)
...
end
end
If a before_add
callback throws an exception, the object does not get added to the collection. Similarly, if a before_remove
callback throws an exception, the object does not get removed from the collection.
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 Customer < ActiveRecord::Base
has_many :orders do
def find_by_order_prefix(order_number)
find_by(region_id: order_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 Customer < ActiveRecord::Base
has_many :orders, -> { extending FindRecentExtension }
end
class Supplier < ActiveRecord::Base
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
.