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A Guide to Active Record Associations

This guide covers the association features of Active Record. By referring to this guide, you will be able to:

1 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.find_by_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.

2 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 association:

  • 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.

2.1 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

belongs_to Association Diagram

2.2 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

has_one Association Diagram

2.3 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

The name of the other model is pluralized when declaring a has_many association.

has_many Association Diagram

2.4 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

has_many :through Association Diagram

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

2.5 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 customer 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

has_one :through Association Diagram

2.6 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

has_and_belongs_to_many Association Diagram

2.7 Choosing Between belongs_to and has_one

If you want to set up a 1–1 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 self.up 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 def self.down drop_table :accounts drop_table :suppliers end end

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.

2.8 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.

2.9 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 self.up create_table :pictures do |t| t.string :name t.integer :imageable_id t.string :imageable_type t.timestamps end end def self.down drop_table :pictures end end

This migration can be simplified by using the t.references form:

class CreatePictures < ActiveRecord::Migration def self.up create_table :pictures do |t| t.string :name t.references :imageable, :polymorphic => true t.timestamps end end def self.down drop_table :pictures end end

Polymorphic Association Diagram

2.10 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.

3 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

3.1 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

3.2 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.

3.3 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.

3.3.1 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 self.up create_table :orders do |t| t.datetime :order_date t.string :order_number t.integer :customer_id end end def self.down drop_table :orders 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.

3.3.2 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.

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 CreateAssemblyPartJoinTable < ActiveRecord::Migration def self.up create_table :assemblies_parts, :id => false do |t| t.integer :assembly_id t.integer :part_id end end def self.down drop_table :assemblies_parts 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 behaviour in a has_and_belongs_to_many association like mangled models IDs, or exceptions about conflicting IDs chances are you forgot that bit.

3.4 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

4 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.

4.1 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.

4.1.1 Methods Added by belongs_to

When you declare a belongs_to association, the declaring class automatically gains four methods related to the association:

  • association(force_reload = false)
  • association=(associate)
  • build_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
4.1.1.1 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.

4.1.1.2 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
4.1.1.3 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")
4.1.1.4 create_association(attributes = {})

The create_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. In addition, the associated object will be saved (assuming that it passes any validations).

@customer = @order.create_customer(:customer_number => 123, :customer_name => "John Doe")
4.1.2 Options for belongs_to

In many situations, you can use the default behavior of belongs_to without any customization. But despite Rails’ emphasis of convention over customization, you can alter that behavior in a number of ways. This section covers the options that you can pass when you create a belongs_to association. For example, an association with several options might look like this:

class Order < ActiveRecord::Base belongs_to :customer, :counter_cache => true, :conditions => "active = 1" end

The belongs_to association supports these options:

  • :autosave
  • :class_name
  • :conditions
  • :counter_cache
  • :dependent
  • :foreign_key
  • :include
  • :polymorphic
  • :readonly
  • :select
  • :validate
4.1.2.1 :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.

4.1.2.2 :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
4.1.2.3 :conditions

The :conditions option lets you specify the conditions that the associated object must meet (in the syntax used by a SQL WHERE clause).

class Order < ActiveRecord::Base belongs_to :customer, :conditions => "active = 1" end
4.1.2.4 :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.

4.1.2.5 :dependent

If you set the :dependent option to :destroy, then deleting this object will call the destroy method on the associated object to delete that object. If you set the :dependent option to :delete, then deleting this object will delete the associated object without calling its destroy method.

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.

4.1.2.6 :foreign_key

By convention, Rails guesses 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

In any case, Rails will not create foreign key columns for you. You need to explicitly define them as part of your migrations.

4.1.2.7 :include

You can use the :include option 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, :include => :customer end class Order < ActiveRecord::Base belongs_to :customer has_many :line_items end class Customer < ActiveRecord::Base has_many :orders end

There’s no need to use :include for immediate associations – that is, if you have Order belongs_to :customer, then the customer is eager-loaded automatically when it’s needed.

4.1.2.8 :polymorphic

Passing true to the :polymorphic option indicates that this is a polymorphic association. Polymorphic associations were discussed in detail earlier in this guide.

4.1.2.9 :readonly

If you set the :readonly option to true, then the associated object will be read-only when retrieved via the association.

4.1.2.10 :select

The :select option lets you override the SQL SELECT clause that is used to retrieve data about the associated object. By default, Rails retrieves all columns.

If you set the :select option on a belongs_to association, you should also set the foreign_key option to guarantee the correct results.

4.1.2.11 :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.

4.1.3 How To Know Whether There’s an Associated Object?

To know whether there’s and associated object just check association.nil?:

if @order.customer.nil? @msg = "No customer found for this order" end
4.1.4 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.

4.2 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.

4.2.1 Methods Added by has_one

When you declare a has_one association, the declaring class automatically gains four methods related to the association:

  • association(force_reload = false)
  • association=(associate)
  • build_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
4.2.1.1 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.

4.2.1.2 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
4.2.1.3 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")
4.2.1.4 create_association(attributes = {})

The create_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. In addition, the associated object will be saved (assuming that it passes any validations).

@account = @supplier.create_account(:terms => "Net 30")
4.2.2 Options for has_one

In many situations, you can use the default behavior of has_one without any customization. But despite Rails’ emphasis of convention over customization, you can alter that behavior in a number of ways. This section covers the options that you can pass when you create a has_one association. For example, an association with several options might look like this:

class Supplier < ActiveRecord::Base has_one :account, :class_name => "Billing", :dependent => :nullify end

The has_one association supports these options:

  • :as
  • :autosave
  • :class_name
  • :conditions
  • :dependent
  • :foreign_key
  • :include
  • :order
  • :primary_key
  • :readonly
  • :select
  • :source
  • :source_type
  • :through
  • :validate
4.2.2.1 :as

Setting the :as option indicates that this is a polymorphic association. Polymorphic associations were discussed in detail earlier in this guide.

4.2.2.2 :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.

4.2.2.3 :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
4.2.2.4 :conditions

The :conditions option lets you specify the conditions that the associated object must meet (in the syntax used by a SQL WHERE clause).

class Supplier < ActiveRecord::Base has_one :account, :conditions => "confirmed = 1" end
4.2.2.5 :dependent

If you set the :dependent option to :destroy, then deleting this object will call the destroy method on the associated object to delete that object. If you set the :dependent option to :delete, then deleting this object will delete the associated object without calling its destroy method. If you set the :dependent option to :nullify, then deleting this object will set the foreign key in the association object to NULL.

4.2.2.6 :foreign_key

By convention, Rails guesses 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

In any case, Rails will not create foreign key columns for you. You need to explicitly define them as part of your migrations.

4.2.2.7 :include

You can use the :include option 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, :include => :representative end class Account < ActiveRecord::Base belongs_to :supplier belongs_to :representative end class Representative < ActiveRecord::Base has_many :accounts end
4.2.2.8 :order

The :order option dictates the order in which associated objects will be received (in the syntax used by a SQL ORDER BY clause). Because a has_one association will only retrieve a single associated object, this option should not be needed.

4.2.2.9 :primary_key

By convention, Rails guesses 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.

4.2.2.10 :readonly

If you set the :readonly option to true, then the associated object will be read-only when retrieved via the association.

4.2.2.11 :select

The :select option lets you override the SQL SELECT clause that is used to retrieve data about the associated object. By default, Rails retrieves all columns.

4.2.2.12 :source

The :source option specifies the source association name for a has_one :through association.

4.2.2.13 :source_type

The :source_type option specifies the source association type for a has_one :through association that proceeds through a polymorphic association.

4.2.2.14 :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.

4.2.2.15 :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.

4.2.3 How To Know Whether There’s an Associated Object?

To know whether there’s and associated object just check association.nil?:

if @supplier.account.nil? @msg = "No account found for this supplier" end
4.2.4 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.

4.3 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.

4.3.1 Methods Added

When you declare a has_many association, the declaring class automatically gains 13 methods related to the association:

  • collection(force_reload = false)
  • collection<<(object, …)
  • collection.delete(object, …)
  • collection=objects
  • collection_singular_ids
  • collection_singular_ids=ids
  • collection.clear
  • collection.empty?
  • collection.size
  • collection.find(…)
  • collection.exist?(…)
  • collection.build(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=objects order_ids order_ids=ids orders.clear orders.empty? orders.size orders.find(...) orders.exist?(...) orders.build(attributes = {}, ...) orders.create(attributes = {})
4.3.1.1 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
4.3.1.2 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
4.3.1.3 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)

Objects will be in addition destroyed if they’re associated with :dependent => :destroy, and deleted if they’re associated with :dependent => :delete_all.

4.3.1.4 collection=objects

The collection= method makes the collection contain only the supplied objects, by adding and deleting as appropriate.

4.3.1.5 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
4.3.1.6 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.

4.3.1.7 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.

4.3.1.8 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 %>
4.3.1.9 collection.size

The collection.size method returns the number of objects in the collection.

@order_count = @customer.orders.size
4.3.1.10 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(:all, :conditions => "open = 1")
4.3.1.11 collection.exist?(…)

The collection.exist? method checks whether an object meeting the supplied conditions exists in the collection. It uses the same syntax and options as ActiveRecord::Base.exists?.

4.3.1.12 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")
4.3.1.13 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 the associated object will be saved (assuming that it passes any validations).

@order = @customer.orders.create(:order_date => Time.now, :order_number => "A12345")
4.3.2 Options for has_many

In many situations, you can use the default behavior for has_many without any customization. But you can alter that behavior in a number of ways. This section covers the options that you can pass when you create a has_many association. For example, an association with several options might look like this:

class Customer < ActiveRecord::Base has_many :orders, :dependent => :delete_all, :validate => :false end

The has_many association supports these options:

  • :as
  • :autosave
  • :class_name
  • :conditions
  • :counter_sql
  • :dependent
  • :extend
  • :finder_sql
  • :foreign_key
  • :group
  • :include
  • :limit
  • :offset
  • :order
  • :primary_key
  • :readonly
  • :select
  • :source
  • :source_type
  • :through
  • :uniq
  • :validate
4.3.2.1 :as

Setting the :as option indicates that this is a polymorphic association, as discussed earlier in this guide.

4.3.2.2 :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.

4.3.2.3 :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
4.3.2.4 :conditions

The :conditions option lets you specify the conditions that the associated object must meet (in the syntax used by a SQL WHERE clause).

class Customer < ActiveRecord::Base has_many :confirmed_orders, :class_name => "Order", :conditions => "confirmed = 1" end

You can also set conditions via a hash:

class Customer < ActiveRecord::Base has_many :confirmed_orders, :class_name => "Order", :conditions => { :confirmed => true } end

If you use a hash-style :conditions 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.

4.3.2.5 :counter_sql

Normally Rails automatically generates the proper SQL to count the association members. With the :counter_sql option, you can specify a complete SQL statement to count them yourself.

If you specify :finder_sql but not :counter_sql, then the counter SQL will be generated by substituting SELECT COUNT(*) FROM for the SELECT ... FROM clause of your :finder_sql statement.

4.3.2.6 :dependent

If you set the :dependent option to :destroy, then deleting this object will call the destroy method on the associated objects to delete those objects. If you set the :dependent option to :delete_all, then deleting this object will delete the associated objects without calling their destroy method. If you set the :dependent option to :nullify, then deleting this object will set the foreign key in the associated objects to NULL.

This option is ignored when you use the :through option on the association.

4.3.2.7 :extend

The :extend option specifies a named module to extend the association proxy. Association extensions are discussed in detail later in this guide.

4.3.2.8 :finder_sql

Normally Rails automatically generates the proper SQL to fetch the association members. With the :finder_sql option, you can specify a complete SQL statement to fetch them yourself. If fetching objects requires complex multi-table SQL, this may be necessary.

4.3.2.9 :foreign_key

By convention, Rails guesses 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

In any case, Rails will not create foreign key columns for you. You need to explicitly define them as part of your migrations.

4.3.2.10 :group

The :group option 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, :through => :orders, :group => "orders.id" end
4.3.2.11 :include

You can use the :include option 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, :include => :line_items end class Order < ActiveRecord::Base belongs_to :customer has_many :line_items end class LineItem < ActiveRecord::Base belongs_to :order end
4.3.2.12 :limit

The :limit option lets you restrict the total number of objects that will be fetched through an association.

class Customer < ActiveRecord::Base has_many :recent_orders, :class_name => "Order", :order => "order_date DESC", :limit => 100 end
4.3.2.13 :offset

The :offset option 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.

4.3.2.14 :order

The :order option dictates the order in which associated objects will be received (in the syntax used by a SQL ORDER BY clause).

class Customer < ActiveRecord::Base has_many :orders, :order => "date_confirmed DESC" end
4.3.2.15 :primary_key

By convention, Rails guesses 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.

4.3.2.16 :readonly

If you set the :readonly option to true, then the associated objects will be read-only when retrieved via the association.

4.3.2.17 :select

The :select option lets you override the SQL SELECT clause that is used to retrieve data about the associated objects. By default, Rails retrieves all columns.

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.

4.3.2.18 :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.

4.3.2.19 :source_type

The :source_type option specifies the source association type for a has_many :through association that proceeds through a polymorphic association.

4.3.2.20 :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.

4.3.2.21 :uniq

Specify the :uniq => true option to remove duplicates from the collection. This is most useful in conjunction with the :through option.

4.3.2.22 :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.

4.3.3 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.

4.4 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.

4.4.1 Methods Added

When you declare a has_and_belongs_to_many association, the declaring class automatically gains 13 methods related to the association:

  • collection(force_reload = false)
  • collection<<(object, …)
  • collection.delete(object, …)
  • collection=objects
  • collection_singular_ids
  • collection_singular_ids=ids
  • collection.clear
  • collection.empty?
  • collection.size
  • collection.find(…)
  • collection.exist?(…)
  • collection.build(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=objects assembly_ids assembly_ids=ids assemblies.clear assemblies.empty? assemblies.size assemblies.find(...) assemblies.exist?(...) assemblies.build(attributes = {}, ...) assemblies.create(attributes = {})
4.4.1.1 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.

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.

4.4.1.2 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
4.4.1.3 collection<<(object, …)

The collection<< method adds one or more objects to the collection by creating records in the join table.

@part.assemblies << @assembly1

This method is aliased as collection.concat and collection.push.

4.4.1.4 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)
4.4.1.5 collection=objects

The collection= method makes the collection contain only the supplied objects, by adding and deleting as appropriate.

4.4.1.6 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
4.4.1.7 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.

4.4.1.8 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.

4.4.1.9 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 %>
4.4.1.10 collection.size

The collection.size method returns the number of objects in the collection.

@assembly_count = @part.assemblies.size
4.4.1.11 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.

@new_assemblies = @part.assemblies.find(:all, :conditions => ["created_at > ?", 2.days.ago])
4.4.1.12 collection.exist?(…)

The collection.exist? method checks whether an object meeting the supplied conditions exists in the collection. It uses the same syntax and options as ActiveRecord::Base.exists?.

4.4.1.13 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"})
4.4.1.14 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 the associated object will be saved (assuming that it passes any validations).

@assembly = @part.assemblies.create( {:assembly_name => "Transmission housing"})
4.4.2 Options for has_and_belongs_to_many

In many situations, you can use the default behavior for has_and_belongs_to_many without any customization. But you can alter that behavior in a number of ways. This section covers the options that you can pass when you create a has_and_belongs_to_many association. For example, an association with several options might look like this:

class Parts < ActiveRecord::Base has_and_belongs_to_many :assemblies, :uniq => true, :read_only => true end

The has_and_belongs_to_many association supports these options:

  • :association_foreign_key
  • :autosave
  • :class_name
  • :conditions
  • :counter_sql
  • :delete_sql
  • :extend
  • :finder_sql
  • :foreign_key
  • :group
  • :include
  • :insert_sql
  • :join_table
  • :limit
  • :offset
  • :order
  • :readonly
  • :select
  • :uniq
  • :validate
4.4.2.1 :association_foreign_key

By convention, Rails guesses 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:

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
4.4.2.2 :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.

4.4.2.3 :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
4.4.2.4 :conditions

The :conditions option lets you specify the conditions that the associated object must meet (in the syntax used by a SQL WHERE clause).

class Parts < ActiveRecord::Base has_and_belongs_to_many :assemblies, :conditions => "factory = 'Seattle'" end

You can also set conditions via a hash:

class Parts < ActiveRecord::Base has_and_belongs_to_many :assemblies, :conditions => { :factory => 'Seattle' } end

If you use a hash-style :conditions option, 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”.

4.4.2.5 :counter_sql

Normally Rails automatically generates the proper SQL to count the association members. With the :counter_sql option, you can specify a complete SQL statement to count them yourself.

If you specify :finder_sql but not :counter_sql, then the counter SQL will be generated by substituting SELECT COUNT(*) FROM for the SELECT ... FROM clause of your :finder_sql statement.

4.4.2.6 :delete_sql

Normally Rails automatically generates the proper SQL to remove links between the associated classes. With the :delete_sql option, you can specify a complete SQL statement to delete them yourself.

4.4.2.7 :extend

The :extend option specifies a named module to extend the association proxy. Association extensions are discussed in detail later in this guide.

4.4.2.8 :finder_sql

Normally Rails automatically generates the proper SQL to fetch the association members. With the :finder_sql option, you can specify a complete SQL statement to fetch them yourself. If fetching objects requires complex multi-table SQL, this may be necessary.

4.4.2.9 :foreign_key

By convention, Rails guesses 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
4.4.2.10 :group

The :group option 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
4.4.2.11 :include

You can use the :include option to specify second-order associations that should be eager-loaded when this association is used.

4.4.2.12 :insert_sql

Normally Rails automatically generates the proper SQL to create links between the associated classes. With the :insert_sql option, you can specify a complete SQL statement to insert them yourself.

4.4.2.13 :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.

4.4.2.14 :limit

The :limit option 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
4.4.2.15 :offset

The :offset option 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.

4.4.2.16 :order

The :order option dictates the order in which associated objects will be received (in the syntax used by a SQL ORDER BY clause).

class Parts < ActiveRecord::Base has_and_belongs_to_many :assemblies, :order => "assembly_name ASC" end
4.4.2.17 :readonly

If you set the :readonly option to true, then the associated objects will be read-only when retrieved via the association.

4.4.2.18 :select

The :select option lets you override the SQL SELECT clause that is used to retrieve data about the associated objects. By default, Rails retrieves all columns.

4.4.2.19 :uniq

Specify the :uniq => true option to remove duplicates from the collection.

4.4.2.20 :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.

4.4.3 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.

4.5 Association Callbacks

Normal callbacks hook into the lifecycle 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 lifecycle 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.

4.6 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 find(:all, :conditions => ["created_at > ?", 5.days.ago]) end end class Customer < ActiveRecord::Base has_many :orders, :extend => FindRecentExtension end class Supplier < ActiveRecord::Base has_many :deliveries, :extend => FindRecentExtension end

To include more than one extension module in a single association, specify an array of modules:

class Customer < ActiveRecord::Base has_many :orders, :extend => [FindRecentExtension, FindActiveExtension] end

Extensions can refer to the internals of the association proxy using these three accessors:

  • proxy_owner returns the object that the association is a part of.
  • proxy_reflection returns the reflection object that describes the association.
  • proxy_target returns the associated object for belongs_to or has_one, or the collection of associated objects for has_many or has_and_belongs_to_many.

5 Changelog

Lighthouse ticket

  • February 1, 2009: Added :autosave option Mike Gunderloy
  • September 28, 2008: Corrected has_many :through diagram, added polymorphic diagram, some reorganization by Mike Gunderloy . First release version.
  • September 22, 2008: Added diagrams, misc. cleanup by Mike Gunderloy (not yet approved for publication)
  • September 14, 2008: initial version by Mike Gunderloy (not yet approved for publication)