Module: ActiveRecord::Associations::ClassMethods

Defined in:: lib/active_record/associations.rb

Overview

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
#other.nil?                       |     X      |      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     |    
#other_ids                        |   X   |    X     |    X
#other_ids=(id,id,...)            |   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     |    
#others.create!(attributes={})    |   X   |    X     |    X
#others.size                      |   X   |    X     |    X
#others.length                    |   X   |    X     |    X
#others.count                     |       |    X     |    X
#others.sum(args*,&block)         |   X   |    X     |    X
#others.empty?                    |   X   |    X     |    X
#others.clear                     |   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.uniq                      |   X   |    X     |    
#others.reset                     |   X   |    X     |    X

Cardinality and associations

ActiveRecord associations can be used to describe relations with one-to-one, one-to-many and many-to-many cardinality. Each model uses an association to describe its role in the relation. In each case, the belongs_to association is 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.

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 along with it in a single SQL call. 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 1. Example:

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

Consider the following loop using the class above:

for post in Post.find(: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, so the find will now weave in a join something like this: LEFT OUTER JOIN authors ON authors.id = posts.author_id. Doing so will cut down the number of queries from 201 to 101.

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

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

That’ll add another join along the lines of: LEFT OUTER JOIN comments ON comments.post_id = posts.id. And we’ll be down to 1 query.

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 the eager loading pulls from multiple tables, you’ll have to disambiguate any column references in both conditions and orders. So :order => "posts.id DESC" will work while :order => "id DESC" will not. Because eager loading generates the SELECT statement too, the :select option is ignored.

You can use eager loading on multiple associations from the same table, but you cannot use those associations in orders and conditions as there is currently not any way to disambiguate them. Eager loading will not pull additional attributes on join tables, so “rich associations” with has_and_belongs_to_many are not a good fit for eager loading.

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.

Table Aliasing

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

Acts as tree example:

TreeMixin.find :all, :include => :children
# => SELECT ... FROM mixins LEFT OUTER JOIN mixins childrens_mixins ...
TreeMixin.find :all, :include => {:children => :parent} # using cascading eager includes
# => SELECT ... FROM mixins LEFT OUTER JOIN mixins childrens_mixins ... 
                            LEFT OUTER JOIN parents_mixins ...
TreeMixin.find :all, :include => {:children => {:parent => :children}} 
# => SELECT ... FROM mixins LEFT OUTER JOIN mixins childrens_mixins ... 
                            LEFT OUTER JOIN parents_mixins ... 
                            LEFT OUTER JOIN mixins childrens_mixins_2

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

Post.find :all, :include => :categories
# => SELECT ... FROM posts LEFT OUTER JOIN categories_posts ... LEFT OUTER JOIN categories ...
Post.find :all, :include => {:categories => :posts}
# => SELECT ... FROM posts LEFT OUTER JOIN categories_posts ... LEFT OUTER JOIN categories ...
                           LEFT OUTER JOIN categories_posts posts_categories_join LEFT OUTER JOIN posts posts_categories
Post.find :all, :include => {:categories => {:posts => :categories}}
# => SELECT ... FROM posts LEFT OUTER JOIN categories_posts ... LEFT OUTER JOIN categories ...
                           LEFT OUTER JOIN categories_posts posts_categories_join LEFT OUTER JOIN posts posts_categories
                           LEFT OUTER JOIN categories_posts categories_posts_join LEFT OUTER JOIN categories categories_posts

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, :include => :comments, :joins => "inner join comments ..."
# => SELECT ... FROM posts LEFT OUTER JOIN comments_posts ON ... INNER JOIN comments ...
Post.find :all, :include => [:comments, :special_comments], :joins => "inner join comments ..."
# => SELECT ... FROM posts LEFT OUTER JOIN comments comments_posts ON ... 
                           LEFT OUTER 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 Company < ActiveRecord::Base; end
  end
end

When Firm#clients is called, it will in turn call MyApplication::Business::Company.find(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.

Defined Under Namespace

Classes: InnerJoinDependency, JoinDependency

Instance Method Summary collapse

#belongs_to(association_id, options = {}) ⇒ Object

Adds the following methods for retrieval and query for a single associated object for which this object holds an id: association is replaced with the symbol passed as the first argument, so belongs_to :author would add among others author.nil?.
#has_and_belongs_to_many(association_id, options = {}, &extension) ⇒ Object

Associates two classes via an intermediate join table.
#has_many(association_id, options = {}, &extension) ⇒ Object

Adds the following methods for retrieval and query of collections of associated objects: collection is replaced with the symbol passed as the first argument, so has_many :clients would add among others clients.empty?.
#has_one(association_id, options = {}) ⇒ Object

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.

Instance Method Details

#belongs_to(association_id, options = {}) ⇒ `Object`

Adds the following methods for retrieval and query for a single associated object for which this object holds an id: association is replaced with the symbol passed as the first argument, so belongs_to :author would add among others author.nil?.

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.
association.nil? - returns true if there is no associated 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.
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).

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#author.nil?
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 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 :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.
: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 the associated class in lower-case and _id suffixed. So a Person class that makes a belongs_to association to a Boss class will use boss_id as the default foreign_key.
: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).

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

# File 'lib/active_record/associations.rb', line 794

def belongs_to(association_id, options = {})
  reflection = create_belongs_to_reflection(association_id, options)
  
  if reflection.options[:polymorphic]
    association_accessor_methods(reflection, BelongsToPolymorphicAssociation)

    module_eval do
      before_save <<-EOF
        association = instance_variable_get("@#{reflection.name}")
        if association && association.target
          if association.new_record?
            association.save(true)
          end
          
          if association.updated?
            self["#{reflection.primary_key_name}"] = association.id
            self["#{reflection.options[:foreign_type]}"] = association.class.base_class.name.to_s
          end
        end
      EOF
    end
  else
    association_accessor_methods(reflection, BelongsToAssociation)
    association_constructor_method(:build,  reflection, BelongsToAssociation)
    association_constructor_method(:create, reflection, BelongsToAssociation)

    module_eval do
      before_save <<-EOF
        association = instance_variable_get("@#{reflection.name}")
        if !association.nil? 
          if association.new_record?
            association.save(true)
          end
          
          if association.updated?
            self["#{reflection.primary_key_name}"] = association.id
          end
        end            
      EOF
    end
  end

  # Create the callbacks to update counter cache
  if options[:counter_cache]
    cache_column = options[:counter_cache] == true ?
      "#{self.to_s.underscore.pluralize}_count" :
      options[:counter_cache]

    module_eval(
      "after_create '#{reflection.name}.class.increment_counter(\"#{cache_column}\", #{reflection.primary_key_name})" +
      " unless #{reflection.name}.nil?'"
    )

    module_eval(
      "before_destroy '#{reflection.name}.class.decrement_counter(\"#{cache_column}\", #{reflection.primary_key_name})" +
      " unless #{reflection.name}.nil?'"
    )
    
    module_eval(
      "#{reflection.class_name}.send(:attr_readonly,\"#{cache_column}\".intern) if defined?(#{reflection.class_name}) && #{reflection.class_name}.respond_to?(:attr_readonly)"
    )
  end
end

#has_and_belongs_to_many(association_id, options = {}, &extension) ⇒ `Object`

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.

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

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.
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).

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#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 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_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 will use person_id as the default foreign_key.
:association_foreign_key - specify the association foreign key used for the association. By default this is guessed to be the name of the associated class in lower-case and _id suffixed. So if the associated class is Project, the has_and_belongs_to_many 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.
: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 set to 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
: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.
: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.

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 :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 'lib/active_record/associations.rb', line 948

def has_and_belongs_to_many(association_id, options = {}, &extension)
  reflection = create_has_and_belongs_to_many_reflection(association_id, options, &extension)
  
  add_multiple_associated_save_callbacks(reflection.name)
  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 <<-end_eval unless method_defined?(old_method)
    alias_method :#{old_method}, :destroy_without_callbacks
    def destroy_without_callbacks
      #{reflection.name}.clear
      #{old_method}
    end
  end_eval

  add_association_callbacks(reflection.name, options)
end

#has_many(association_id, options = {}, &extension) ⇒ `Object`

Adds the following methods for retrieval and query of collections of associated objects: collection is replaced with the symbol passed as the first argument, so has_many :clients would add among others clients.empty?.

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.

This will also destroy the objects if they’re declared as belongs_to and dependent on this model.
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 Base.find.
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!

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.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.

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_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%'.
: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.
: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.
: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.
: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.
: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 set to true, duplicates will be omitted from the collection. Useful in conjunction with :through.

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 :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 'lib/active_record/associations.rb', line 666

def has_many(association_id, options = {}, &extension)
  reflection = create_has_many_reflection(association_id, options, &extension)

  configure_dependency_for_has_many(reflection)

  if options[:through]
    collection_reader_method(reflection, HasManyThroughAssociation)
    collection_accessor_methods(reflection, HasManyThroughAssociation, false)
  else
    add_multiple_associated_save_callbacks(reflection.name)
    add_association_callbacks(reflection.name, reflection.options)
    collection_accessor_methods(reflection, HasManyAssociation)
  end
end

#has_one(association_id, options = {}) ⇒ `Object`

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

# File 'lib/active_record/associations.rb', line 726

def has_one(association_id, options = {})
  reflection = create_has_one_reflection(association_id, options)

  module_eval do
    after_save <<-EOF
      association = instance_variable_get("@#{reflection.name}")
      if !association.nil? && (new_record? || association.new_record? || association["#{reflection.primary_key_name}"] != id)
        association["#{reflection.primary_key_name}"] = id
        association.save(true)
      end
    EOF
  end

  association_accessor_methods(reflection, HasOneAssociation)
  association_constructor_method(:build,  reflection, HasOneAssociation)
  association_constructor_method(:create, reflection, HasOneAssociation)
  
  configure_dependency_for_has_one(reflection)
end

Module: ActiveRecord::Associations::ClassMethods

Overview

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

Defined Under Namespace

Instance Method Summary collapse

Instance Method Details

#belongs_to(association_id, options = {}) ⇒ Object

#has_and_belongs_to_many(association_id, options = {}, &extension) ⇒ Object

#has_many(association_id, options = {}, &extension) ⇒ Object

#has_one(association_id, options = {}) ⇒ Object

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

Type safety with `ActiveRecord::AssociationTypeMismatch`

#belongs_to(association_id, options = {}) ⇒ `Object`

#has_and_belongs_to_many(association_id, options = {}, &extension) ⇒ `Object`

#has_many(association_id, options = {}, &extension) ⇒ `Object`

#has_one(association_id, options = {}) ⇒ `Object`