Module: ActiveRecord::Associations::ClassMethods

Defined in:
lib/active_record/associations.rb,
lib/active_record/deprecated_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 was 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 to ease the traversel and manipulation of its relationships:

  • Project#portfolio, Project#portfolio=(portfolio), Project#portfolio.nil?, Project#portfolio?(portfolio)

  • Project#project_manager, Project#project_manager=(project_manager), Project#project_manger.nil?, Project#project_manager?(project_manager), Project#build_project_manager, Project#create_project_manager

  • Project#milestones.empty?, Project#milestones.size, Project#milestones, Project#milestones<<(milestone), Project#milestones.delete(milestone), Project#milestones.find(milestone_id), Project#milestones.find_all(conditions), Project#milestones.build, Project#milestones.create

  • Project#categories.empty?, Project#categories.size, Project#categories, Project#categories<<(category1), Project#categories.delete(category1)

Example

Is it belongs_to or has_one?

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

class Post < ActiveRecord::Base
  has_one :author
end

class Author < ActiveRecord::Base
  belongs_to :post
end

The tables for these classes could look something like:

CREATE TABLE posts (
  id int(11) NOT NULL auto_increment,
  title varchar default NULL,
  PRIMARY KEY  (id)
)

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

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

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’ll 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, such as:

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 a ActiveRecord::AssociationTypeMismatch.

Options

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

Instance Method Summary collapse

Instance Method Details

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

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

  • 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?(object, force_reload = false) - returns true if the object is of the same type and has the same id as the associated object.

  • association.nil? - returns true if there’s an associated object.

Example: An Post class declares has_one :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?

The declaration can also include an options hash to specialize the generated methods.

Options are:

  • :class_name - specify the class name of the association. Use it only if that name can’t be infered 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 from which the associated object will be picked at the top. Specified 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 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).

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}'



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# File 'lib/active_record/associations.rb', line 302

def belongs_to(association_id, options = {})
  validate_options([ :class_name, :foreign_key, :remote, :conditions, :order, :dependent, :counter_cache ], options.keys)

  association_name, association_class_name, class_primary_key_name =
      associate_identification(association_id, options[:class_name], options[:foreign_key], false)

  association_class_primary_key_name = options[:foreign_key] || Inflector.underscore(Inflector.demodulize(association_class_name)) + "_id"

  if options[:remote]
    association_finder = <<-"end_eval"
      #{association_class_name}.find_first(
        "#{class_primary_key_name} = '\#{id}'#{options[:conditions] ? " AND " + options[:conditions] : ""}",
        #{options[:order] ? "\"" + options[:order] + "\"" : "nil" }
      )
    end_eval
  else
    association_finder = options[:conditions] ?
      "#{association_class_name}.find_on_conditions(#{association_class_primary_key_name}, \"#{options[:conditions]}\")" :
      "#{association_class_name}.find(#{association_class_primary_key_name})"
  end

  has_association_method(association_name)
  association_reader_method(association_name, association_finder)
  belongs_to_writer_method(association_name, association_class_name, association_class_primary_key_name)
  association_comparison_method(association_name, association_class_name)

  if options[:counter_cache]
    module_eval(
      "after_create '#{association_class_name}.increment_counter(\"#{Inflector.pluralize(self.to_s.downcase). + "_count"}\", #{association_class_primary_key_name})" +
      " if has_#{association_name}?'"
    )

    module_eval(
      "before_destroy '#{association_class_name}.decrement_counter(\"#{Inflector.pluralize(self.to_s.downcase) + "_count"}\", #{association_class_primary_key_name})" +
      " if has_#{association_name}?'"
    )          
  end
end

#deprecated_add_association_relation(association_name) ⇒ Object

:nodoc:



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# File 'lib/active_record/deprecated_associations.rb', line 13

def deprecated_add_association_relation(association_name)# :nodoc:
  module_eval <<-"end_eval", __FILE__, __LINE__
    def add_#{association_name}(*items)
      #{association_name}.concat(items)
    end
  end_eval
end

#deprecated_build_method(collection_name) ⇒ Object

:nodoc:



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# File 'lib/active_record/deprecated_associations.rb', line 61

def deprecated_build_method(collection_name)# :nodoc:
  module_eval <<-"end_eval", __FILE__, __LINE__
    def build_to_#{collection_name}(attributes = {})
      #{collection_name}.build(attributes)
    end
  end_eval
end

#deprecated_collection_count_method(collection_name) ⇒ Object

:nodoc:



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# File 'lib/active_record/deprecated_associations.rb', line 4

def deprecated_collection_count_method(collection_name)# :nodoc:
  module_eval <<-"end_eval", __FILE__, __LINE__
    def #{collection_name}_count(force_reload = false)
      #{collection_name}.reload if force_reload
      #{collection_name}.size
    end
  end_eval
end

#deprecated_create_method(collection_name) ⇒ Object

:nodoc:



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# File 'lib/active_record/deprecated_associations.rb', line 53

def deprecated_create_method(collection_name)# :nodoc:
  module_eval <<-"end_eval", __FILE__, __LINE__
    def create_in_#{collection_name}(attributes = {})
      #{collection_name}.create(attributes)
    end
  end_eval
end

#deprecated_find_all_in_collection_method(collection_name) ⇒ Object

:nodoc:



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# File 'lib/active_record/deprecated_associations.rb', line 45

def deprecated_find_all_in_collection_method(collection_name)# :nodoc:
  module_eval <<-"end_eval", __FILE__, __LINE__
    def find_all_in_#{collection_name}(runtime_conditions = nil, orderings = nil, limit = nil, joins = nil)
      #{collection_name}.find_all(runtime_conditions, orderings, limit, joins)
    end
  end_eval
end

#deprecated_find_in_collection_method(collection_name) ⇒ Object

:nodoc:



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# File 'lib/active_record/deprecated_associations.rb', line 37

def deprecated_find_in_collection_method(collection_name)# :nodoc:
  module_eval <<-"end_eval", __FILE__, __LINE__
    def find_in_#{collection_name}(association_id)
      #{collection_name}.find(association_id)
    end
  end_eval
end

#deprecated_has_collection_method(collection_name) ⇒ Object

:nodoc:



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# File 'lib/active_record/deprecated_associations.rb', line 29

def deprecated_has_collection_method(collection_name)# :nodoc:
  module_eval <<-"end_eval", __FILE__, __LINE__
    def has_#{collection_name}?(force_reload = false)
      !#{collection_name}(force_reload).empty?
    end
  end_eval
end

#deprecated_remove_association_relation(association_name) ⇒ Object

:nodoc:



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# File 'lib/active_record/deprecated_associations.rb', line 21

def deprecated_remove_association_relation(association_name)# :nodoc:
  module_eval <<-"end_eval", __FILE__, __LINE__
    def remove_#{association_name}(items)
      #{association_name}.delete(items)
    end
  end_eval
end

#has_and_belongs_to_many(association_id, options = {}) ⇒ 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”. Adds the following methods for retrival and query. collection is replaced with the symbol passed as the first argument, so has_and_belongs_to_many :categories would add among others add_categories.

  • collection(force_reload = false) - returns an array of all the associated objects. An empty array is returned if none is found.

  • !collection.empty? - returns true if there’s any associated objects.

  • collection.size - returns the number of associated objects.

  • collection<<(object) - adds an association between this object and the object given as argument. Multiple associations can be created by passing an array of objects instead.

  • collection.delete(object) - removes the association between this object and the object given as argument. Multiple associations can be removed by passing an array of objects instead.

Example: An Developer class declares has_and_belongs_to_many :projects, which will add:

  • Developer#projects

  • !Developer#projects.empty?

  • Developer#projects.size

  • Developer#projects<<

  • Developer#projects.delete

The declaration can also include an options hash to specialize the generated methods.

Options are:

  • :class_name - specify the class name of the association. Use it only if that name can’t be infered 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 the associated class is Project that makes a has_and_belongs_to_many association will use “project_id” as the default association foreign_key.

  • :order - specify the order in which the associated objects are returned as a “ORDER BY” sql fragment, such as “last_name, first_name DESC”.

  • :finder_sql - overwrite the default generated SQL used to fetch the association with a manual one

  • :delete_sql - overwrite the default generated SQL used to remove links between the associated classes with a manual one

  • :insert_sql - overwrite the default generated SQL used to add links between the associated classes with a manual one

Option examples:

has_and_belongs_to_many :projects
has_and_belongs_to_many :nations, :class_name => "Country"
has_and_belongs_to_many :categories, :join_table => "prods_cats"



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# File 'lib/active_record/associations.rb', line 388

def has_and_belongs_to_many(association_id, options = {})
  validate_options([ :class_name, :table_name, :foreign_key, :association_foreign_key,
                     :join_table, :finder_sql, :delete_sql, :insert_sql, :order ], options.keys)
  association_name, association_class_name, association_class_primary_key_name =
        associate_identification(association_id, options[:class_name], options[:foreign_key])

  join_table  = options[:join_table] || 
    join_table_name(undecorated_table_name(self.to_s), undecorated_table_name(association_class_name))
 
  
  module_eval <<-"end_eval", __FILE__, __LINE__
    def #{association_name}(force_reload = false)
      if @#{association_name}.nil?
          @#{association_name} = HasAndBelongsToManyCollection.new(self, "#{association_name}", "#{association_class_name}", 
            "#{association_class_primary_key_name}", '#{join_table}', #{options.inspect})
      end
      @#{association_name}.reload if force_reload
      
      return @#{association_name}
    end
  end_eval

  before_destroy_sql = "DELETE FROM #{join_table} WHERE #{Inflector.foreign_key(self.class_name)} = '\\\#{self.id}'"
  module_eval(%{before_destroy "self.connection.delete(%{#{before_destroy_sql}})"}) # "
  
  # deprecated api
  deprecated_collection_count_method(association_name)
  deprecated_add_association_relation(association_name)
  deprecated_remove_association_relation(association_name)
  deprecated_has_collection_method(association_name)
end

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

Adds the following methods for retrival 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 has_clients?.

  • collection(force_reload = false) - returns an array of all the associated objects. An empty array is returned if none is found.

  • collection<<(object) - adds the object to the collection (by setting the foreign key on it) and saves it.

  • collection.delete(object) - removes the association by setting the foreign key to null on the associated object.

  • !collection.empty? - returns true if there’s any 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.find_all(conditions = nil, orderings = nil, limit = nil, joins = nil) - finds all associated objects responding criterias mentioned (like in the standard find_all) 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 a foreign key but has not yet been saved.

  • collection.create(attributes = {}) - returns a new object of the collection type that has been instantiated with attributes and linked to this object through a foreign key and that has already been saved (if it passed the validation).

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

  • Firm#clients (similar to Clients.find_all "firm_id = #{id}")

  • Firm#clients<<

  • Firm#clients.delete

  • !Firm#clients.empty? (similar to firm.clients.length > 0)

  • Firm#clients.size (similar to Client.count "firm_id = #{id}")

  • Firm#clients.find (similar to Client.find_on_conditions(id, "firm_id = #{id}"))

  • Firm#clients.find_all (similar to Client.find_all "firm_id = #{id}")

  • Firm#clients.build (similar to Client.new("firm_id" => id))

  • Firm#clients.create (similar to c = Client.new("client_id" => id); c.save; c)

The declaration can also include an options hash to specialize the generated methods.

Options are:

  • :class_name - specify the class name of the association. Use it only if that name can’t be infered 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 a “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 true all the associated object are destroyed alongside this object

  • :exclusively_dependent - if set to true all the associated object are deleted in one SQL statement without having their before_destroy callback run. This should only be used on associations that depend solely on this class and don’t need to do any clean-up in before_destroy. The upside is that it’s much faster, especially if there’s a counter_cache involved.

  • :finder_sql - specify a complete SQL statement to fetch the association. This is a good way to go for complex associations that depends on multiple tables. Note: When this option is used, find_in_collection is not added.

Option examples:

has_many :comments, :order => "posted_on"
has_many :people, :class_name => "Person", :conditions => "deleted = 0", :order => "name"
has_many :tracks, :order => "position", :dependent => true
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'



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# File 'lib/active_record/associations.rb', line 175

def has_many(association_id, options = {})
  validate_options([ :foreign_key, :class_name, :exclusively_dependent, :dependent, :conditions, :order, :finder_sql ], options.keys)
  association_name, association_class_name, association_class_primary_key_name =
        associate_identification(association_id, options[:class_name], options[:foreign_key])
 
  if options[:dependent]
    module_eval "before_destroy '#{association_name}.each { |o| o.destroy }'"
  end

  if options[:exclusively_dependent]
    module_eval "before_destroy Proc.new{ |record| #{association_class_name}.delete_all(%(#{association_class_primary_key_name} = '\#{record.id}')) }"
  end

  module_eval <<-"end_eval", __FILE__, __LINE__
    def #{association_name}(force_reload = false)
      if @#{association_name}.nil?
          @#{association_name} = HasManyAssociation.new(self, "#{association_name}", "#{association_class_name}", 
            "#{association_class_primary_key_name}", #{options.inspect})
      end
      @#{association_name}.reload if force_reload
      
      return @#{association_name}
    end
  end_eval
  
  # deprecated api
  deprecated_collection_count_method(association_name)
  deprecated_add_association_relation(association_name)
  deprecated_remove_association_relation(association_name)
  deprecated_has_collection_method(association_name)
  deprecated_find_in_collection_method(association_name)
  deprecated_find_all_in_collection_method(association_name)
  deprecated_create_method(association_name)
  deprecated_build_method(association_name)
end

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

Adds the following methods for retrival and query of a single associated object. association is replaced with the symbol passed as the first argument, so has_one :manager would add among others has_manager?.

  • association(force_reload = false) - returns the associated object. Nil is returned if none is found.

  • association=(associate) - assigns the associate object, extracts the primary key, sets it as the foreign key, and saves the associate object.

  • association?(object, force_reload = false) - returns true if the object is of the same type and has the same id as the associated object.

  • !association.nil? - returns true if there’s an 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 and linked to this object through a foreign key and that has already been saved (if it passed the validation).

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

  • Account#beneficiary (similar to Beneficiary.find_first "account_id = #{id}")

  • Account#beneficiary=(beneficiary) (similar to beneficiary.account_id = account.id; beneficiary.save)

  • Account#beneficiary? (similar to account.beneficiary == some_beneficiary)

  • !Account#beneficiary.nil?

  • Account#build_beneficiary (similar to Beneficiary.new("account_id" => id))

  • Account#create_beneficiary (similar to b = Beneficiary.new("account_id" => id); b.save; b)

The declaration can also include an options hash to specialize the generated methods.

Options are:

  • :class_name - specify the class name of the association. Use it only if that name can’t be infered from the association name. So has_one :manager will by default be linked to the Manager class, but if the real class name is Person, you’ll have to specify it with this option.

  • :conditions - specify the conditions that the associated object must meet in order to be included as a “WHERE” sql fragment, such as “rank = 5”.

  • :order - specify the order from which the associated object will be picked at the top. Specified as

    an "ORDER BY" sql fragment, such as "last_name, first_name DESC"

  • :dependent - if set to true the associated object is destroyed alongside this object

  • :foreign_key - specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and “_id” suffixed. So a Person class that makes a has_one association will use “person_id” as the default foreign_key.

Option examples:

has_one :credit_card, :dependent => true
has_one :last_comment, :class_name => "Comment", :order => "posted_on"
has_one :project_manager, :class_name => "Person", :conditions => "role = 'project_manager'"



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# File 'lib/active_record/associations.rb', line 251

def has_one(association_id, options = {})
  options.merge!({ :remote => true })
  belongs_to(association_id, options)

  association_name, association_class_name, class_primary_key_name =
      associate_identification(association_id, options[:class_name], options[:foreign_key], false)

  has_one_writer_method(association_name, association_class_name, class_primary_key_name)
  build_method("build_", association_name, association_class_name, class_primary_key_name)
  create_method("create_", association_name, association_class_name, class_primary_key_name)
  
  module_eval "before_destroy '#{association_name}.destroy if has_#{association_name}?'" if options[:dependent]
end