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, attributesand connectionwould 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_toassociation is always used in the model that has the foreign key.

One-to-one

Use has_onein the base, and belongs_toin 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_manyin the base, and belongs_toin 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_manyassociation with the :throughoption 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_manyin 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_manywhen working with legacy schemas or when you never work directly with the relationship itself.

Is it a belongs_toor has_oneassociation?

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_torelationship. 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 set the :autosave option on a has_one, belongs_to, has_many, or has_and_belongs_to_manyassociation. Setting it to truewill alwayssave the members, whereas setting it to falsewill neversave the members.

One-to-one associations

  • Assigning an object to a has_oneassociation 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 falseand the assignment is cancelled.

  • If you wish to assign an object to a has_oneassociation without saving it, use the association.buildmethod (documented below).

  • Assigning an object to a belongs_toassociation 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_manyor 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 pushor similar) fails, then pushreturns false.

  • You can add an object to a collection without automatically saving it by using the collection.buildmethod (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_removeand after_remove.

Should any of the before_addcallbacks throw an exception, the object does not get added to the collection. Same with the before_removecallbacks; 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 :extendoption. 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_toand has_one, or the collection of associated objects for has_manyand 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_manyassociation. 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_manyassociation 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.

Similarly you can go through a has_oneassociation on the join model:

class Group < ActiveRecord::Base
  has_many   :users
  has_many   :avatars, :through => :users
end
class User < ActiveRecord::Base
  belongs_to :group
  has_one    :avatar
end
class Avatar < ActiveRecord::Base
  belongs_to :user
end
@group = Group.first
@group.users.collect { |u| u.avatar }.flatten # select all avatars for all users in the group
@group.avatars                                # selects all avatars by going through the User join model.

An important caveat with going through has_oneor has_manyassociations on the join model is that these associations are read-only. For example, the following would not work following the previous example:

@group.avatars << Avatar.new                # this would work if User belonged_to Avatar rather than the other way around.
@group.avatars.delete(@group.avatars.last)  # so would this

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_manyassociation 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_idinteger column and an attachable_typestring 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 typecolumn 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_toassociation that also used the :authorsymbol. After loading the posts, find will collect the author_idfrom 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.idand 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 :includean 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/ JOINclauses 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 _joinsuffix:

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#clientsis 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.

Namespace
Methods
B
H
Instance Public methods
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_oneinstead. See also ActiveRecord::Associations::ClassMethods's overview on when to use has_oneand 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. nilis 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 attributesand 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).

( associationis replaced with the symbol passed as the first argument, so belongs_to :authorwould 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 WHERESQL 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 :personassociation 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”.

:primary_key

Specify the method that returns the primary key of associated object used for the association. By default this is id.

:dependent

If set to :destroy, the associated object is destroyed when this object is. If set to :delete, the associated object is deleted withoutcalling its destroy method. This option should not be specified when belongs_tois used in conjunction with a has_manyrelationship 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_counterand 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_countfor 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/ falsevalue 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_readonlylist 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. falseby default.

:autosave

If true, always save the associated object or destroy it if marked for destruction, when saving the parent object. Off by default.

:touch

If true, the associated object will be touched (the updated_at/on attributes set to now) when this record is either saved or destroyed. If you specify a symbol, that attribute will be updated with the current time instead of the updated_at/on attribute.

Option examples:

belongs_to :firm, :foreign_key => "client_of"
belongs_to :person, :primary_key => "name", :foreign_key => "person_name"
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
belongs_to :company, :touch => true
belongs_to :company, :touch => :employees_last_updated_at
# File activerecord/lib/active_record/associations.rb, line 1038
def belongs_to(association_id, options = {})
  reflection = create_belongs_to_reflection(association_id, options)
  if reflection.options[:polymorphic]
    association_accessor_methods(reflection, BelongsToPolymorphicAssociation)
  else
    association_accessor_methods(reflection, BelongsToAssociation)
    association_constructor_method(:build,  reflection, BelongsToAssociation)
    association_constructor_method(:create, reflection, BelongsToAssociation)
  end
  add_counter_cache_callbacks(reflection)          if options[:counter_cache]
  add_touch_callbacks(reflection, options[:touch]) if options[:touch]
  configure_dependency_for_belongs_to(reflection)
end
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_tableoption 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_manyassociations. 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.pushand 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 trueif there are no associated objects.

collection.size

Returns the number of associated objects.

collection.find(id)

Finds an associated object responding to the idand 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 attributesand 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).

( collectionis replaced with the symbol passed as the first argument, so has_and_belongs_to_many :categorieswould 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 :projectswill 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_manyassociation 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_manyassociation 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 WHERESQL 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.createor @blog.posts.build.

:order

Specify the order in which the associated objects are returned as an ORDER BYSQL 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_sqlis specified but not :counter_sql, :counter_sqlwill 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 BYSQL-clause.

:having

Combined with :groupthis can be used to filter the records that a GROUP BYreturns. 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. trueby 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}'
# File activerecord/lib/active_record/associations.rb, line 1215
      def has_and_belongs_to_many(association_id, options = {}, &extension)
        reflection = create_has_and_belongs_to_many_reflection(association_id, options, &extension)
        collection_accessor_methods(reflection, HasAndBelongsToManyAssociation)
        # Don't use a before_destroy callback since users' before_destroy
        # callbacks will be executed after the association is wiped out.
        old_method = "destroy_without_habtm_shim_for_#{reflection.name}"
        class_eval "          alias_method :#{old_method}, :destroy_without_callbacks  # alias_method :destroy_without_habtm_shim_for_posts, :destroy_without_callbacks
          def destroy_without_callbacks                            # def destroy_without_callbacks
            #{reflection.name}.clear                               #   posts.clear
            #{old_method}                                          #   destroy_without_habtm_shim_for_posts
          end                                                      # end
" unless method_defined?(old_method)
        add_association_callbacks(reflection.name, options)
      end
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 trueif 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 attributesand 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: collectionis replaced with the symbol passed as the first argument, so has_many :clientswould 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 WHERESQL 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.createor @blog.posts.build.

:order

Specify the order in which the associated objects are returned as an ORDER BYSQL 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_manyassociation 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 :destroyall the associated objects are destroyed alongside this object by calling their destroymethod. If set to :delete_allall associated objects are deleted withoutcalling their destroymethod. If set to :nullifyall associated objects' foreign keys are set to NULL withoutcalling their savecallbacks. Warning:This option is ignored when also using the :throughoption.

: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_collectionis not added.

:counter_sql

Specify a complete SQL statement to fetch the size of the association. If :finder_sqlis specified but not :counter_sql, :counter_sqlwill 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 BYSQL-clause.

:having

Combined with :groupthis can be used to filter the records that a GROUP BYreturns. 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_nameand :foreign_keyare ignored, as the association uses the source reflection. You can only use a :throughquery through a belongs_to has_oneor has_manyassociation 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 :subscribersor :subscriberon Subscription, unless a :sourceis 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'
# File activerecord/lib/active_record/associations.rb, line 828
def has_many(association_id, options = {}, &extension)
  reflection = create_has_many_reflection(association_id, options, &extension)
  configure_dependency_for_has_many(reflection)
  add_association_callbacks(reflection.name, reflection.options)
  if options[:through]
    collection_accessor_methods(reflection, HasManyThroughAssociation)
  else
    collection_accessor_methods(reflection, HasManyAssociation)
  end
end
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. nilis 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 attributesand 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).

( associationis replaced with the symbol passed as the first argument, so has_one :managerwould 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 WHERESQL fragment, such as rank = 5 . Record creation from the association is scoped if a hash is used. has_one :account, :conditions => {:enabled => true}will create an enabled account with @company.create_accountor @company.build_account.

:order

Specify the order in which the associated objects are returned as an ORDER BYSQL 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 withoutcalling 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_oneassociation 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_nameand :foreign_keyare ignored, as the association uses the source reflection. You can only use a :throughquery through a has_oneor belongs_toassociation 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 => :favoriteswill look for a :favoriteon Favorite, unless a :sourceis 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. falseby 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
# File activerecord/lib/active_record/associations.rb, line 934
def has_one(association_id, options = {})
  if options[:through]
    reflection = create_has_one_through_reflection(association_id, options)
    association_accessor_methods(reflection, ActiveRecord::Associations::HasOneThroughAssociation)
  else
    reflection = create_has_one_reflection(association_id, options)
    association_accessor_methods(reflection, HasOneAssociation)
    association_constructor_method(:build,  reflection, HasOneAssociation)
    association_constructor_method(:create, reflection, HasOneAssociation)
    configure_dependency_for_has_one(reflection)
  end
end