ActiveRecord::Associations::ClassMethods

Inheritance

Associations are a set of macro-like class methods for tying objects together through foreign keys. They express relationships like "Project has one Project Manager" or "Project belongs to a Portfolio". Each macro adds a number of methods to the class which are specialized according to the collection or association symbol and the options hash. It works much the same way as Ruby‘s own attr* methods. Example:

  class Project < ActiveRecord::Base
    belongs_to              :portfolio
    has_one                 :project_manager
    has_many                :milestones
    has_and_belongs_to_many :categories
  end

The project class now has the following methods (and more) to ease the traversal and manipulation of its relationships:

Project#portfolio, Project#portfolio=(portfolio), Project#portfolio.nil?
Project#project_manager, Project#project_manager=(project_manager), Project#project_manager.nil?,
Project#milestones.empty?, Project#milestones.size, Project#milestones, Project#milestones<<(milestone), Project#milestones.delete(milestone), Project#milestones.find(milestone_id), Project#milestones.find(:all, options), Project#milestones.build, Project#milestones.create
Project#categories.empty?, Project#categories.size, Project#categories, Project#categories<<(category1), Project#categories.delete(category1)

A word of warning

Don‘t create associations that have the same name as instance methods of ActiveRecord::Base. Since the association adds a method with that name to its model, it will override the inherited method and break things. For instance, attributes and connection would be bad choices for association names.

Auto-generated methods

Singular associations (one-to-one)

                                    |            |  belongs_to  |
  generated methods                 | belongs_to | :polymorphic | has_one
  ----------------------------------+------------+--------------+---------
  other                             |     X      |      X       |    X
  other=(other)                     |     X      |      X       |    X
  build_other(attributes={})        |     X      |              |    X
  create_other(attributes={})       |     X      |              |    X
  other.create!(attributes={})      |            |              |    X

Collection associations (one-to-many / many-to-many)

                                    |       |          | has_many
  generated methods                 | habtm | has_many | :through
  ----------------------------------+-------+----------+----------
  others                            |   X   |    X     |    X
  others=(other,other,...)          |   X   |    X     |    X
  other_ids                         |   X   |    X     |    X
  other_ids=(id,id,...)             |   X   |    X     |    X
  others<<                          |   X   |    X     |    X
  others.push                       |   X   |    X     |    X
  others.concat                     |   X   |    X     |    X
  others.build(attributes={})       |   X   |    X     |    X
  others.create(attributes={})      |   X   |    X     |    X
  others.create!(attributes={})     |   X   |    X     |    X
  others.size                       |   X   |    X     |    X
  others.length                     |   X   |    X     |    X
  others.count                      |   X   |    X     |    X
  others.sum(args*,&block)          |   X   |    X     |    X
  others.empty?                     |   X   |    X     |    X
  others.clear                      |   X   |    X     |    X
  others.delete(other,other,...)    |   X   |    X     |    X
  others.delete_all                 |   X   |    X     |
  others.destroy_all                |   X   |    X     |    X
  others.find(*args)                |   X   |    X     |    X
  others.find_first                 |   X   |          |
  others.exists?                    |   X   |    X     |    X
  others.uniq                       |   X   |    X     |    X
  others.reset                      |   X   |    X     |    X

Cardinality and associations

Active Record associations can be used to describe one-to-one, one-to-many and many-to-many relationships between models. Each model uses an association to describe its role in the relation. The belongs_to association is always used in the model that has the foreign key.

One-to-one

Use has_one in the base, and belongs_to in the associated model.

  class Employee < ActiveRecord::Base
    has_one :office
  end
  class Office < ActiveRecord::Base
    belongs_to :employee    # foreign key - employee_id
  end

One-to-many

Use has_many in the base, and belongs_to in the associated model.

  class Manager < ActiveRecord::Base
    has_many :employees
  end
  class Employee < ActiveRecord::Base
    belongs_to :manager     # foreign key - manager_id
  end

Many-to-many

There are two ways to build a many-to-many relationship.

The first way uses a has_many association with the :through option and a join model, so there are two stages of associations.

  class Assignment < ActiveRecord::Base
    belongs_to :programmer  # foreign key - programmer_id
    belongs_to :project     # foreign key - project_id
  end
  class Programmer < ActiveRecord::Base
    has_many :assignments
    has_many :projects, :through => :assignments
  end
  class Project < ActiveRecord::Base
    has_many :assignments
    has_many :programmers, :through => :assignments
  end

For the second way, use has_and_belongs_to_many in both models. This requires a join table that has no corresponding model or primary key.

  class Programmer < ActiveRecord::Base
    has_and_belongs_to_many :projects       # foreign keys in the join table
  end
  class Project < ActiveRecord::Base
    has_and_belongs_to_many :programmers    # foreign keys in the join table
  end

Choosing which way to build a many-to-many relationship is not always simple. If you need to work with the relationship model as its own entity, use has_many :through. Use has_and_belongs_to_many when working with legacy schemas or when you never work directly with the relationship itself.

Is it a `belongs_to` or `has_one` association?

Both express a 1-1 relationship. The difference is mostly where to place the foreign key, which goes on the table for the class declaring the belongs_to relationship. Example:

  class User < ActiveRecord::Base
    # I reference an account.
    belongs_to :account
  end

  class Account < ActiveRecord::Base
    # One user references me.
    has_one :user
  end

The tables for these classes could look something like:

  CREATE TABLE users (
    id int(11) NOT NULL auto_increment,
    account_id int(11) default NULL,
    name varchar default NULL,
    PRIMARY KEY  (id)
  )

  CREATE TABLE accounts (
    id int(11) NOT NULL auto_increment,
    name varchar default NULL,
    PRIMARY KEY  (id)
  )

Unsaved objects and associations

You can manipulate objects and associations before they are saved to the database, but there is some special behavior you should be aware of, mostly involving the saving of associated objects.

Unless you enable the :autosave option on a has_one, belongs_to, has_many, or has_and_belongs_to_many association, in which case the members are always saved.

One-to-one associations

Assigning an object to a has_one association automatically saves that object and the object being replaced (if there is one), in order to update their primary keys - except if the parent object is unsaved (new_record? == true).
If either of these saves fail (due to one of the objects being invalid) the assignment statement returns false and the assignment is cancelled.
If you wish to assign an object to a has_one association without saving it, use the association.build method (documented below).
Assigning an object to a belongs_to association does not save the object, since the foreign key field belongs on the parent. It does not save the parent either.

Collections

Adding an object to a collection (has_many or has_and_belongs_to_many) automatically saves that object, except if the parent object (the owner of the collection) is not yet stored in the database.
If saving any of the objects being added to a collection (via push or similar) fails, then push returns false.
You can add an object to a collection without automatically saving it by using the collection.build method (documented below).
All unsaved (new_record? == true) members of the collection are automatically saved when the parent is saved.

Association callbacks

Similar to the normal callbacks that hook into the lifecycle of an Active Record object, you can also define callbacks that get triggered when you add an object to or remove an object from an association collection. Example:

  class Project
    has_and_belongs_to_many :developers, :after_add => :evaluate_velocity

    def evaluate_velocity(developer)
      ...
    end
  end

It‘s possible to stack callbacks by passing them as an array. Example:

  class Project
    has_and_belongs_to_many :developers, :after_add => [:evaluate_velocity, Proc.new { |p, d| p.shipping_date = Time.now}]
  end

Possible callbacks are: before_add, after_add, before_remove and after_remove.

Should any of the before_add callbacks throw an exception, the object does not get added to the collection. Same with the before_remove callbacks; if an exception is thrown the object doesn‘t get removed.

Association extensions

The proxy objects that control the access to associations can be extended through anonymous modules. This is especially beneficial for adding new finders, creators, and other factory-type methods that are only used as part of this association. Example:

  class Account < ActiveRecord::Base
    has_many :people do
      def find_or_create_by_name(name)
        first_name, last_name = name.split(" ", 2)
        find_or_create_by_first_name_and_last_name(first_name, last_name)
      end
    end
  end

  person = Account.find(:first).people.find_or_create_by_name("David Heinemeier Hansson")
  person.first_name # => "David"
  person.last_name  # => "Heinemeier Hansson"

If you need to share the same extensions between many associations, you can use a named extension module. Example:

  module FindOrCreateByNameExtension
    def find_or_create_by_name(name)
      first_name, last_name = name.split(" ", 2)
      find_or_create_by_first_name_and_last_name(first_name, last_name)
    end
  end

  class Account < ActiveRecord::Base
    has_many :people, :extend => FindOrCreateByNameExtension
  end

  class Company < ActiveRecord::Base
    has_many :people, :extend => FindOrCreateByNameExtension
  end

If you need to use multiple named extension modules, you can specify an array of modules with the :extend option. In the case of name conflicts between methods in the modules, methods in modules later in the array supercede those earlier in the array. Example:

  class Account < ActiveRecord::Base
    has_many :people, :extend => [FindOrCreateByNameExtension, FindRecentExtension]
  end

Some extensions can only be made to work with knowledge of the association proxy‘s internals. Extensions can access relevant state using accessors on the association proxy:

proxy_owner - Returns the object the association is part of.
proxy_reflection - Returns the reflection object that describes the association.
proxy_target - Returns the associated object for belongs_to and has_one, or the collection of associated objects for has_many and has_and_belongs_to_many.

Association Join Models

Has Many associations can be configured with the :through option to use an explicit join model to retrieve the data. This operates similarly to a has_and_belongs_to_many association. The advantage is that you‘re able to add validations, callbacks, and extra attributes on the join model. Consider the following schema:

  class Author < ActiveRecord::Base
    has_many :authorships
    has_many :books, :through => :authorships
  end

  class Authorship < ActiveRecord::Base
    belongs_to :author
    belongs_to :book
  end

  @author = Author.find :first
  @author.authorships.collect { |a| a.book } # selects all books that the author's authorships belong to.
  @author.books                              # selects all books by using the Authorship join model

You can also go through a has_many association on the join model:

  class Firm < ActiveRecord::Base
    has_many   :clients
    has_many   :invoices, :through => :clients
  end

  class Client < ActiveRecord::Base
    belongs_to :firm
    has_many   :invoices
  end

  class Invoice < ActiveRecord::Base
    belongs_to :client
  end

  @firm = Firm.find :first
  @firm.clients.collect { |c| c.invoices }.flatten # select all invoices for all clients of the firm
  @firm.invoices                                   # selects all invoices by going through the Client join model.

Polymorphic Associations

Polymorphic associations on models are not restricted on what types of models they can be associated with. Rather, they specify an interface that a has_many association must adhere to.

  class Asset < ActiveRecord::Base
    belongs_to :attachable, :polymorphic => true
  end

  class Post < ActiveRecord::Base
    has_many :assets, :as => :attachable         # The :as option specifies the polymorphic interface to use.
  end

  @asset.attachable = @post

This works by using a type column in addition to a foreign key to specify the associated record. In the Asset example, you‘d need an attachable_id integer column and an attachable_type string column.

Using polymorphic associations in combination with single table inheritance (STI) is a little tricky. In order for the associations to work as expected, ensure that you store the base model for the STI models in the type column of the polymorphic association. To continue with the asset example above, suppose there are guest posts and member posts that use the posts table for STI. In this case, there must be a type column in the posts table.

  class Asset < ActiveRecord::Base
    belongs_to :attachable, :polymorphic => true

    def attachable_type=(sType)
       super(sType.to_s.classify.constantize.base_class.to_s)
    end
  end

  class Post < ActiveRecord::Base
    # because we store "Post" in attachable_type now :dependent => :destroy will work
    has_many :assets, :as => :attachable, :dependent => :destroy
  end

  class GuestPost < Post
  end

  class MemberPost < Post
  end

Caching

All of the methods are built on a simple caching principle that will keep the result of the last query around unless specifically instructed not to. The cache is even shared across methods to make it even cheaper to use the macro-added methods without worrying too much about performance at the first go. Example:

  project.milestones             # fetches milestones from the database
  project.milestones.size        # uses the milestone cache
  project.milestones.empty?      # uses the milestone cache
  project.milestones(true).size  # fetches milestones from the database
  project.milestones             # uses the milestone cache

Eager loading of associations

Eager loading is a way to find objects of a certain class and a number of named associations. This is one of the easiest ways of to prevent the dreaded 1+N problem in which fetching 100 posts that each need to display their author triggers 101 database queries. Through the use of eager loading, the 101 queries can be reduced to 2. Example:

  class Post < ActiveRecord::Base
    belongs_to :author
    has_many   :comments
  end

Consider the following loop using the class above:

  for post in Post.all
    puts "Post:            " + post.title
    puts "Written by:      " + post.author.name
    puts "Last comment on: " + post.comments.first.created_on
  end

To iterate over these one hundred posts, we‘ll generate 201 database queries. Let‘s first just optimize it for retrieving the author:

  for post in Post.find(:all, :include => :author)

This references the name of the belongs_to association that also used the :author symbol. After loading the posts, find will collect the author_id from each one and load all the referenced authors with one query. Doing so will cut down the number of queries from 201 to 102.

We can improve upon the situation further by referencing both associations in the finder with:

  for post in Post.find(:all, :include => [ :author, :comments ])

This will load all comments with a single query. This reduces the total number of queries to 3. More generally the number of queries will be 1 plus the number of associations named (except if some of the associations are polymorphic belongs_to - see below).

To include a deep hierarchy of associations, use a hash:

  for post in Post.find(:all, :include => [ :author, { :comments => { :author => :gravatar } } ])

That‘ll grab not only all the comments but all their authors and gravatar pictures. You can mix and match symbols, arrays and hashes in any combination to describe the associations you want to load.

All of this power shouldn‘t fool you into thinking that you can pull out huge amounts of data with no performance penalty just because you‘ve reduced the number of queries. The database still needs to send all the data to Active Record and it still needs to be processed. So it‘s no catch-all for performance problems, but it‘s a great way to cut down on the number of queries in a situation as the one described above.

Since only one table is loaded at a time, conditions or orders cannot reference tables other than the main one. If this is the case Active Record falls back to the previously used LEFT OUTER JOIN based strategy. For example

  Post.find(:all, :include => [ :author, :comments ], :conditions => ['comments.approved = ?', true])

will result in a single SQL query with joins along the lines of: LEFT OUTER JOIN comments ON comments.post_id = posts.id and LEFT OUTER JOIN authors ON authors.id = posts.author_id. Note that using conditions like this can have unintended consequences. In the above example posts with no approved comments are not returned at all, because the conditions apply to the SQL statement as a whole and not just to the association. You must disambiguate column references for this fallback to happen, for example :order => "author.name DESC" will work but :order => "name DESC" will not.

If you do want eagerload only some members of an association it is usually more natural to :include an association which has conditions defined on it:

  class Post < ActiveRecord::Base
    has_many :approved_comments, :class_name => 'Comment', :conditions => ['approved = ?', true]
  end

  Post.find(:all, :include => :approved_comments)

will load posts and eager load the approved_comments association, which contains only those comments that have been approved.

If you eager load an association with a specified :limit option, it will be ignored, returning all the associated objects:

  class Picture < ActiveRecord::Base
    has_many :most_recent_comments, :class_name => 'Comment', :order => 'id DESC', :limit => 10
  end

  Picture.find(:first, :include => :most_recent_comments).most_recent_comments # => returns all associated comments.

When eager loaded, conditions are interpolated in the context of the model class, not the model instance. Conditions are lazily interpolated before the actual model exists.

Eager loading is supported with polymorphic associations.

  class Address < ActiveRecord::Base
    belongs_to :addressable, :polymorphic => true
  end

A call that tries to eager load the addressable model

  Address.find(:all, :include => :addressable)

will execute one query to load the addresses and load the addressables with one query per addressable type. For example if all the addressables are either of class Person or Company then a total of 3 queries will be executed. The list of addressable types to load is determined on the back of the addresses loaded. This is not supported if Active Record has to fallback to the previous implementation of eager loading and will raise ActiveRecord::EagerLoadPolymorphicError. The reason is that the parent model‘s type is a column value so its corresponding table name cannot be put in the FROM/JOIN clauses of that query.

Table Aliasing

Active Record uses table aliasing in the case that a table is referenced multiple times in a join. If a table is referenced only once, the standard table name is used. The second time, the table is aliased as #{reflection_name}_#{parent_table_name}. Indexes are appended for any more successive uses of the table name.

  Post.find :all, :joins => :comments
  # => SELECT ... FROM posts INNER JOIN comments ON ...
  Post.find :all, :joins => :special_comments # STI
  # => SELECT ... FROM posts INNER JOIN comments ON ... AND comments.type = 'SpecialComment'
  Post.find :all, :joins => [:comments, :special_comments] # special_comments is the reflection name, posts is the parent table name
  # => SELECT ... FROM posts INNER JOIN comments ON ... INNER JOIN comments special_comments_posts

Acts as tree example:

  TreeMixin.find :all, :joins => :children
  # => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
  TreeMixin.find :all, :joins => {:children => :parent}
  # => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
                              INNER JOIN parents_mixins ...
  TreeMixin.find :all, :joins => {:children => {:parent => :children}}
  # => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
                              INNER JOIN parents_mixins ...
                              INNER JOIN mixins childrens_mixins_2

Has and Belongs to Many join tables use the same idea, but add a _join suffix:

  Post.find :all, :joins => :categories
  # => SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ...
  Post.find :all, :joins => {:categories => :posts}
  # => SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ...
                             INNER JOIN categories_posts posts_categories_join INNER JOIN posts posts_categories
  Post.find :all, :joins => {:categories => {:posts => :categories}}
  # => SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ...
                             INNER JOIN categories_posts posts_categories_join INNER JOIN posts posts_categories
                             INNER JOIN categories_posts categories_posts_join INNER JOIN categories categories_posts_2

If you wish to specify your own custom joins using a :joins option, those table names will take precedence over the eager associations:

  Post.find :all, :joins => :comments, :joins => "inner join comments ..."
  # => SELECT ... FROM posts INNER JOIN comments_posts ON ... INNER JOIN comments ...
  Post.find :all, :joins => [:comments, :special_comments], :joins => "inner join comments ..."
  # => SELECT ... FROM posts INNER JOIN comments comments_posts ON ...
                             INNER JOIN comments special_comments_posts ...
                             INNER JOIN comments ...

Table aliases are automatically truncated according to the maximum length of table identifiers according to the specific database.

Modules

By default, associations will look for objects within the current module scope. Consider:

  module MyApplication
    module Business
      class Firm < ActiveRecord::Base
         has_many :clients
       end

      class Client < ActiveRecord::Base; end
    end
  end

When Firm#clients is called, it will in turn call MyApplication::Business::Client.find_all_by_firm_id(firm.id). If you want to associate with a class in another module scope, this can be done by specifying the complete class name. Example:

  module MyApplication
    module Business
      class Firm < ActiveRecord::Base; end
    end

    module Billing
      class Account < ActiveRecord::Base
        belongs_to :firm, :class_name => "MyApplication::Business::Firm"
      end
    end
  end

Type safety with `ActiveRecord::AssociationTypeMismatch`

If you attempt to assign an object to an association that doesn‘t match the inferred or specified :class_name, you‘ll get an ActiveRecord::AssociationTypeMismatch.

Options

All of the association macros can be specialized through options. This makes cases more complex than the simple and guessable ones possible.

Methods

Instance

Visibility	Signature
public	belongs_to (association_id, options = {})
public	has_and_belongs_to_many (association_id, options = {}, &extension)
public	has_many (association_id, options = {}, &extension)
public	has_one (association_id, options = {})

Instance Method Detail

belongs_to(association_id, options = {})

Specifies a one-to-one association with another class. This method should only be used if this class contains the foreign key. If the other class contains the foreign key, then you should use has_one instead. See also ActiveRecord::Associations::ClassMethods‘s overview on when to use has_one and when to use belongs_to.

Methods will be added for retrieval and query for a single associated object, for which this object holds an id:

association(force_reload = false): Returns the associated object. nil is returned if none is found.
association=(associate): Assigns the associate object, extracts the primary key, and sets it as the foreign key.
build_association(attributes = {}): Returns a new object of the associated type that has been instantiated with attributes and linked to this object through a foreign key, but has not yet been saved.
create_association(attributes = {}): Returns a new object of the associated type that has been instantiated with attributes, linked to this object through a foreign key, and that has already been saved (if it passed the validation).

(association is replaced with the symbol passed as the first argument, so belongs_to :author would add among others author.nil?.)

Example

A Post class declares belongs_to :author, which will add:

Post#author (similar to Author.find(author_id))
Post#author=(author) (similar to post.author_id = author.id)
Post#author? (similar to post.author == some_author)
Post#build_author (similar to post.author = Author.new)
Post#create_author (similar to post.author = Author.new; post.author.save; post.author)

The declaration can also include an options hash to specialize the behavior of the association.

Options

:class_name: Specify the class name of the association. Use it only if that name can‘t be inferred from the association name. So has_one :author will by default be linked to the Author class, but if the real class name is Person, you‘ll have to specify it with this option.
:conditions: Specify the conditions that the associated object must meet in order to be included as a WHERE SQL fragment, such as authorized = 1.
:select: By default, this is * as in SELECT * FROM, but can be changed if, for example, you want to do a join but not include the joined columns. Do not forget to include the primary and foreign keys, otherwise it will raise an error.
:foreign_key: Specify the foreign key used for the association. By default this is guessed to be the name of the association with an "_id" suffix. So a class that defines a belongs_to :person association will use "person_id" as the default :foreign_key. Similarly, belongs_to :favorite_person, :class_name => "Person" will use a foreign key of "favorite_person_id".
:dependent: If set to :destroy, the associated object is destroyed when this object is. If set to :delete, the associated object is deleted without calling its destroy method. This option should not be specified when belongs_to is used in conjunction with a has_many relationship on another class because of the potential to leave orphaned records behind.
:counter_cache: Caches the number of belonging objects on the associate class through the use of increment_counter and decrement_counter. The counter cache is incremented when an object of this class is created and decremented when it‘s destroyed. This requires that a column named #{table_name}_count (such as comments_count for a belonging Comment class) is used on the associate class (such as a Post class). You can also specify a custom counter cache column by providing a column name instead of a true/false value to this option (e.g., :counter_cache => :my_custom_counter.) Note: Specifying a counter cache will add it to that model‘s list of readonly attributes using attr_readonly.
:include: Specify second-order associations that should be eager loaded when this object is loaded.
:polymorphic: Specify this association is a polymorphic association by passing true. Note: If you‘ve enabled the counter cache, then you may want to add the counter cache attribute to the attr_readonly list in the associated classes (e.g. class Post; attr_readonly :comments_count; end).
:readonly: If true, the associated object is readonly through the association.
:validate: If false, don‘t validate the associated objects when saving the parent object. false by default.
:autosave: If true, always save the associated object or destroy it if marked for destruction, when saving the parent object. Off by default.

Option examples:

  belongs_to :firm, :foreign_key => "client_of"
  belongs_to :author, :class_name => "Person", :foreign_key => "author_id"
  belongs_to :valid_coupon, :class_name => "Coupon", :foreign_key => "coupon_id",
             :conditions => 'discounts > #{payments_count}'
  belongs_to :attachable, :polymorphic => true
  belongs_to :project, :readonly => true
  belongs_to :post, :counter_cache => true

has_and_belongs_to_many(association_id, options = {}, &extension)

Specifies a many-to-many relationship with another class. This associates two classes via an intermediate join table. Unless the join table is explicitly specified as an option, it is guessed using the lexical order of the class names. So a join between Developer and Project will give the default join table name of "developers_projects" because "D" outranks "P". Note that this precedence 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". Be aware of this caveat, and use the custom :join_table option if you need to.

The join table should not have a primary key or a model associated with it. You must manually generate the join table with a migration such as this:

  class CreateDevelopersProjectsJoinTable < ActiveRecord::Migration
    def self.up
      create_table :developers_projects, :id => false do |t|
        t.integer :developer_id
        t.integer :project_id
      end
    end

    def self.down
      drop_table :developers_projects
    end
  end

Deprecated: Any additional fields added to the join table will be placed as attributes when pulling records out through has_and_belongs_to_many associations. Records returned from join tables with additional attributes will be marked as readonly (because we can‘t save changes to the additional attributes). It‘s strongly recommended that you upgrade any associations with attributes to a real join model (see introduction).

Adds the following methods for retrieval and query:

collection(force_reload = false): Returns an array of all the associated objects. An empty array is returned if none are found.
collection<<(object, …): Adds one or more objects to the collection by creating associations in the join table (collection.push and collection.concat are aliases to this method).
collection.delete(object, …): Removes one or more objects from the collection by removing their associations from the join table. This does not destroy the objects.
collection=objects: Replaces the collection‘s content by deleting and adding objects as appropriate.
collection_singular_ids: Returns an array of the associated objects’ ids.
collection_singular_ids=ids: Replace the collection by the objects identified by the primary keys in ids.
collection.clear: Removes every object from the collection. This does not destroy the objects.
collection.empty?: Returns true if there are no associated objects.
collection.size: Returns the number of associated objects.
collection.find(id): Finds an associated object responding to the id and that meets the condition that it has to be associated with this object. Uses the same rules as ActiveRecord::Base.find.
collection.exists?(…): Checks whether an associated object with the given conditions exists. Uses the same rules as ActiveRecord::Base.exists?.
collection.build(attributes = {}): Returns a new object of the collection type that has been instantiated with attributes and linked to this object through the join table, but has not yet been saved.
collection.create(attributes = {}): Returns a new object of the collection type that has been instantiated with attributes, linked to this object through the join table, and that has already been saved (if it passed the validation).

(collection is replaced with the symbol passed as the first argument, so has_and_belongs_to_many :categories would add among others categories.empty?.)

Example

A Developer class declares has_and_belongs_to_many :projects, which will add:

Developer#projects
Developer#projects<<
Developer#projects.delete
Developer#projects=
Developer#project_ids
Developer#project_ids=
Developer#projects.clear
Developer#projects.empty?
Developer#projects.size
Developer#projects.find(id)
Developer#clients.exists?(…)
Developer#projects.build (similar to Project.new("project_id" => id))
Developer#projects.create (similar to c = Project.new("project_id" => id); c.save; c)

The declaration may include an options hash to specialize the behavior of the association.

Options

:class_name: Specify the class name of the association. Use it only if that name can‘t be inferred from the association name. So has_and_belongs_to_many :projects will by default be linked to the Project class, but if the real class name is SuperProject, you‘ll have to specify it with this option.
:join_table: Specify the name of the join table if the default based on lexical order isn‘t what you want. WARNING: If you‘re overwriting the table name of either class, the table_name method MUST be declared underneath any has_and_belongs_to_many declaration in order to work.
:foreign_key: Specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and "_id" suffixed. So a Person class that makes a has_and_belongs_to_many association to Project will use "person_id" as the default :foreign_key.
:association_foreign_key: Specify the foreign key used for the association on the receiving side of the association. By default this is guessed to be the name of the associated class in lower-case and "_id" suffixed. So if a Person class makes a has_and_belongs_to_many association to Project, the association will use "project_id" as the default :association_foreign_key.
:conditions: Specify the conditions that the associated object must meet in order to be included as a WHERE SQL fragment, such as authorized = 1. Record creations from the association are scoped if a hash is used. has_many :posts, :conditions => {:published => true} will create published posts with @blog.posts.create or @blog.posts.build.
:order: Specify the order in which the associated objects are returned as an ORDER BY SQL fragment, such as last_name, first_name DESC
:uniq: If true, duplicate associated objects will be ignored by accessors and query methods.
:finder_sql: Overwrite the default generated SQL statement used to fetch the association with a manual statement
:counter_sql: Specify a complete SQL statement to fetch the size of the association. If :finder_sql is specified but not :counter_sql, :counter_sql will be generated by replacing SELECT … FROM with SELECT COUNT(*) FROM.
:delete_sql: Overwrite the default generated SQL statement used to remove links between the associated classes with a manual statement.
:insert_sql: Overwrite the default generated SQL statement used to add links between the associated classes with a manual statement.
:extend: Anonymous module for extending the proxy, see "Association extensions".
:include: Specify second-order associations that should be eager loaded when the collection is loaded.
:group: An attribute name by which the result should be grouped. Uses the GROUP BY SQL-clause.
:having: Combined with +:group+ this can be used to filter the records that a GROUP BY returns. Uses the HAVING SQL-clause.
:limit: An integer determining the limit on the number of rows that should be returned.
:offset: An integer determining the offset from where the rows should be fetched. So at 5, it would skip the first 4 rows.
:select: By default, this is * as in SELECT * FROM, but can be changed if, for example, you want to do a join but not include the joined columns. Do not forget to include the primary and foreign keys, otherwise it will raise an error.
:readonly: If true, all the associated objects are readonly through the association.
:validate: If false, don‘t validate the associated objects when saving the parent object. true by default.
:autosave: If true, always save any loaded members and destroy members marked for destruction, when saving the parent object. Off by default.

Option examples:

  has_and_belongs_to_many :projects
  has_and_belongs_to_many :projects, :include => [ :milestones, :manager ]
  has_and_belongs_to_many :nations, :class_name => "Country"
  has_and_belongs_to_many :categories, :join_table => "prods_cats"
  has_and_belongs_to_many :categories, :readonly => true
  has_and_belongs_to_many :active_projects, :join_table => 'developers_projects', :delete_sql =>
  'DELETE FROM developers_projects WHERE active=1 AND developer_id = #{id} AND project_id = #{record.id}'

has_many(association_id, options = {}, &extension)

Specifies a one-to-many association. The following methods for retrieval and query of collections of associated objects will be added:

collection(force_reload = false): Returns an array of all the associated objects. An empty array is returned if none are found.
collection<<(object, …): Adds one or more objects to the collection by setting their foreign keys to the collection‘s primary key.
collection.delete(object, …): Removes one or more objects from the collection by setting their foreign keys to NULL. Objects will be in addition destroyed if they‘re associated with :dependent => :destroy, and deleted if they‘re associated with :dependent => :delete_all.
collection=objects: Replaces the collections content by deleting and adding objects as appropriate.
collection_singular_ids: Returns an array of the associated objects’ ids
collection_singular_ids=ids: Replace the collection with the objects identified by the primary keys in ids
collection.clear: 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, otherwise sets their foreign keys to NULL.
collection.empty?: Returns true if there are no associated objects.
collection.size: Returns the number of associated objects.
collection.find(...): Finds an associated object according to the same rules as ActiveRecord::Base.find.
collection.exists?(…): Checks whether an associated object with the given conditions exists. Uses the same rules as ActiveRecord::Base.exists?.
collection.build(attributes = {}, …): Returns one or more new objects of the collection type that have been instantiated with attributes and linked to this object through a foreign key, but have not yet been saved. Note: This only works if an associated object already exists, not if it‘s nil!
collection.create(attributes = {}): Returns a new object of the collection type that has been instantiated with attributes, linked to this object through a foreign key, and that has already been saved (if it passed the validation). Note: This only works if an associated object already exists, not if it‘s nil!

(Note: collection is replaced with the symbol passed as the first argument, so has_many :clients would add among others clients.empty?.)

Example

Example: A Firm class declares has_many :clients, which will add:

Firm#clients (similar to Clients.find :all, :conditions => ["firm_id = ?", id])
Firm#clients<<
Firm#clients.delete
Firm#clients=
Firm#client_ids
Firm#client_ids=
Firm#clients.clear
Firm#clients.empty? (similar to firm.clients.size == 0)
Firm#clients.size (similar to Client.count "firm_id = #{id}")
Firm#clients.find (similar to Client.find(id, :conditions => "firm_id = #{id}"))
Firm#clients.exists?(:name => ‘ACME’) (similar to Client.exists?(:name => ‘ACME’, :firm_id => firm.id))
Firm#clients.build (similar to Client.new("firm_id" => id))
Firm#clients.create (similar to c = Client.new("firm_id" => id); c.save; c)

The declaration can also include an options hash to specialize the behavior of the association.

Supported options

:class_name: Specify the class name of the association. Use it only if that name can‘t be inferred from the association name. So has_many :products will by default be linked to the Product class, but if the real class name is SpecialProduct, you‘ll have to specify it with this option.
:conditions: Specify the conditions that the associated objects must meet in order to be included as a WHERE SQL fragment, such as price > 5 AND name LIKE ‘B%’. Record creations from the association are scoped if a hash is used. has_many :posts, :conditions => {:published => true} will create published posts with @blog.posts.create or @blog.posts.build.
:order: Specify the order in which the associated objects are returned as an ORDER BY SQL fragment, such as last_name, first_name DESC.
:foreign_key: Specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and "_id" suffixed. So a Person class that makes a has_many association will use "person_id" as the default :foreign_key.
:primary_key: Specify the method that returns the primary key used for the association. By default this is id.
:dependent: If set to :destroy all the associated objects are destroyed alongside this object by calling their destroy method. If set to :delete_all all associated objects are deleted without calling their destroy method. If set to :nullify all associated objects’ foreign keys are set to NULL without calling their save callbacks. *Warning:* This option is ignored when also using the :through option.
:finder_sql: Specify a complete SQL statement to fetch the association. This is a good way to go for complex associations that depend on multiple tables. Note: When this option is used, find_in_collection is not added.
:counter_sql: Specify a complete SQL statement to fetch the size of the association. If :finder_sql is specified but not :counter_sql, :counter_sql will be generated by replacing SELECT … FROM with SELECT COUNT(*) FROM.
:extend: Specify a named module for extending the proxy. See "Association extensions".
:include: Specify second-order associations that should be eager loaded when the collection is loaded.
:group: An attribute name by which the result should be grouped. Uses the GROUP BY SQL-clause.
:having: Combined with +:group+ this can be used to filter the records that a GROUP BY returns. Uses the HAVING SQL-clause.
:limit: An integer determining the limit on the number of rows that should be returned.
:offset: An integer determining the offset from where the rows should be fetched. So at 5, it would skip the first 4 rows.
:select: By default, this is * as in SELECT * FROM, but can be changed if you, for example, want to do a join but not include the joined columns. Do not forget to include the primary and foreign keys, otherwise it will raise an error.
:as: Specifies a polymorphic interface (See belongs_to).
:through: Specifies a Join Model through which to perform the query. Options for :class_name and :foreign_key are ignored, as the association uses the source reflection. You can only use a :through query through a belongs_to or has_many association on the join model.
:source: Specifies the source association name used by has_many :through queries. Only use it if the name cannot be inferred from the association. has_many :subscribers, :through => :subscriptions will look for either :subscribers or :subscriber on Subscription, unless a :source is given.
:source_type: Specifies type of the source association used by has_many :through queries where the source association is a polymorphic belongs_to.
:uniq: If true, duplicates will be omitted from the collection. Useful in conjunction with :through.
:readonly: If true, all the associated objects are readonly through the association.
:validate: If false, don‘t validate the associated objects when saving the parent object. true by default.
:autosave: If true, always save any loaded members and destroy members marked for destruction, when saving the parent object. Off by default.

Option examples:

  has_many :comments, :order => "posted_on"
  has_many :comments, :include => :author
  has_many :people, :class_name => "Person", :conditions => "deleted = 0", :order => "name"
  has_many :tracks, :order => "position", :dependent => :destroy
  has_many :comments, :dependent => :nullify
  has_many :tags, :as => :taggable
  has_many :reports, :readonly => true
  has_many :subscribers, :through => :subscriptions, :source => :user
  has_many :subscribers, :class_name => "Person", :finder_sql =>
      'SELECT DISTINCT people.* ' +
      'FROM people p, post_subscriptions ps ' +
      'WHERE ps.post_id = #{id} AND ps.person_id = p.id ' +
      'ORDER BY p.first_name'

has_one(association_id, options = {})

Specifies a one-to-one association with another class. This method should only be used if the other class contains the foreign key. If the current class contains the foreign key, then you should use belongs_to instead. See also ActiveRecord::Associations::ClassMethods‘s overview on when to use has_one and when to use belongs_to.

The following methods for retrieval and query of a single associated object will be added:

association(force_reload = false): Returns the associated object. nil is returned if none is found.
association=(associate): Assigns the associate object, extracts the primary key, sets it as the foreign key, and saves the associate object.
build_association(attributes = {}): Returns a new object of the associated type that has been instantiated with attributes and linked to this object through a foreign key, but has not yet been saved. Note: This ONLY works if an association already exists. It will NOT work if the association is nil.
create_association(attributes = {}): Returns a new object of the associated type that has been instantiated with attributes, linked to this object through a foreign key, and that has already been saved (if it passed the validation).

(association is replaced with the symbol passed as the first argument, so has_one :manager would add among others manager.nil?.)

Example

An Account class declares has_one :beneficiary, which will add:

Account#beneficiary (similar to Beneficiary.find(:first, :conditions => "account_id = #{id}"))
Account#beneficiary=(beneficiary) (similar to beneficiary.account_id = account.id; beneficiary.save)
Account#build_beneficiary (similar to Beneficiary.new("account_id" => id))
Account#create_beneficiary (similar to b = Beneficiary.new("account_id" => id); b.save; b)

Options

The declaration can also include an options hash to specialize the behavior of the association.

Options are:

:class_name: Specify the class name of the association. Use it only if that name can‘t be inferred from the association name. So has_one :manager will by default be linked to the Manager class, but if the real class name is Person, you‘ll have to specify it with this option.
:conditions: Specify the conditions that the associated object must meet in order to be included as a WHERE SQL fragment, such as rank = 5.
:order: Specify the order in which the associated objects are returned as an ORDER BY SQL fragment, such as last_name, first_name DESC.
:dependent: If set to :destroy, the associated object is destroyed when this object is. If set to :delete, the associated object is deleted without calling its destroy method. If set to :nullify, the associated object‘s foreign key is set to NULL. Also, association is assigned.
:foreign_key: Specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and "_id" suffixed. So a Person class that makes a has_one association will use "person_id" as the default :foreign_key.
:primary_key: Specify the method that returns the primary key used for the association. By default this is id.
:include: Specify second-order associations that should be eager loaded when this object is loaded.
:as: Specifies a polymorphic interface (See belongs_to).
:select: By default, this is * as in SELECT * FROM, but can be changed if, for example, you want to do a join but not include the joined columns. Do not forget to include the primary and foreign keys, otherwise it will raise an error.
:through: Specifies a Join Model through which to perform the query. Options for :class_name and :foreign_key are ignored, as the association uses the source reflection. You can only use a :through query through a has_one or belongs_to association on the join model.
:source: Specifies the source association name used by has_one :through queries. Only use it if the name cannot be inferred from the association. has_one :favorite, :through => :favorites will look for a :favorite on Favorite, unless a :source is given.
:source_type: Specifies type of the source association used by has_one :through queries where the source association is a polymorphic belongs_to.
:readonly: If true, the associated object is readonly through the association.
:validate: If false, don‘t validate the associated object when saving the parent object. false by default.
:autosave: If true, always save the associated object or destroy it if marked for destruction, when saving the parent object. Off by default.

Option examples:

  has_one :credit_card, :dependent => :destroy  # destroys the associated credit card
  has_one :credit_card, :dependent => :nullify  # updates the associated records foreign key value to NULL rather than destroying it
  has_one :last_comment, :class_name => "Comment", :order => "posted_on"
  has_one :project_manager, :class_name => "Person", :conditions => "role = 'project_manager'"
  has_one :attachment, :as => :attachable
  has_one :boss, :readonly => :true
  has_one :club, :through => :membership
  has_one :primary_address, :through => :addressables, :conditions => ["addressable.primary = ?", true], :source => :addressable

A word of warning

Auto-generated methods

Singular associations (one-to-one)

Collection associations (one-to-many / many-to-many)

Cardinality and associations

One-to-one

One-to-many

Many-to-many

Is it a belongs_to or has_one association?

Unsaved objects and associations

One-to-one associations

Collections

Association callbacks

Association extensions

Association Join Models

Polymorphic Associations

Caching

Eager loading of associations

Table Aliasing

Modules

Type safety with ActiveRecord::AssociationTypeMismatch

Options

Methods

Instance

Instance Method Detail

belongs_to(association_id, options = {})

Example

Options

has_and_belongs_to_many(association_id, options = {}, &extension)

Example

Options

has_many(association_id, options = {}, &extension)

Example

Supported options

has_one(association_id, options = {})

Example

Options

Is it a `belongs_to` or `has_one` association?

Type safety with `ActiveRecord::AssociationTypeMismatch`