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 (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#project_manager.build, Project#project_manager.create
-
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)
)
Unsaved objects and associations
You can manipulate objects and associations before they are saved to the database, but there is some special behaviour 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.
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
-
#belongs_to(association_id, options = {}) ⇒ Object
Adds the following methods for retrieval and query for a single associated object that this object holds an id to.
-
#deprecated_add_association_relation(association_name) ⇒ Object
:nodoc:.
- #deprecated_association_comparison_method(association_name, association_class_name) ⇒ Object
- #deprecated_build_method(method_prefix, collection_name, collection_class_name, class_primary_key_name) ⇒ Object
-
#deprecated_collection_build_method(collection_name) ⇒ Object
:nodoc:.
-
#deprecated_collection_count_method(collection_name) ⇒ Object
:nodoc:.
-
#deprecated_collection_create_method(collection_name) ⇒ Object
:nodoc:.
- #deprecated_create_method(method_prefix, collection_name, collection_class_name, class_primary_key_name) ⇒ Object
-
#deprecated_find_all_in_collection_method(collection_name) ⇒ Object
:nodoc:.
-
#deprecated_find_in_collection_method(collection_name) ⇒ Object
:nodoc:.
- #deprecated_has_association_method(association_name) ⇒ Object
-
#deprecated_has_collection_method(collection_name) ⇒ Object
:nodoc:.
-
#deprecated_remove_association_relation(association_name) ⇒ Object
:nodoc:.
-
#has_and_belongs_to_many(association_id, options = {}) ⇒ Object
Associates two classes via an intermediate join table.
-
#has_many(association_id, options = {}) ⇒ Object
Adds the following methods for retrieval and query of collections of associated objects.
-
#has_one(association_id, options = {}) ⇒ Object
Adds the following methods for retrieval and query of a single associated object.
Instance Method Details
#belongs_to(association_id, options = {}) ⇒ Object
Adds the following methods for retrieval 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 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. -
association.build(attributes = {})
- returns a new object of the associated type that has been instantiated withattributes
and linked to this object through a foreign key but has not yet been saved. -
association.create(attributes = {})
- returns a new object of the associated type that has been instantiated withattributes
and linked to this object through a foreign key and that has already been saved (if it passed the validation).
Example: A Post class declares has_one :author
, which will add:
-
Post#author
(similar toAuthor.find(author_id)
) -
Post#author=(author)
(similar topost.author_id = author.id
) -
Post#author?
(similar topost.author == some_author
) -
Post#author.nil?
-
Post#author.build
(similar toAuthor.new("post_id" => id)
) -
Post#author.create
(similar tob = Author.new("post_id" => id); b.save; b
)
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 infered from the association name. Sohas_one :author
will by default be linked to theAuthor
class, but if the real class name isPerson
, 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 aPerson
class that makes a belongs_to association to aBoss
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 352 def belongs_to(association_id, = {}) ([ :class_name, :foreign_key, :remote, :conditions, :order, :dependent, :counter_cache ], .keys) association_name, association_class_name, class_primary_key_name = associate_identification(association_id, [:class_name], [:foreign_key], false) require_association_class(association_class_name) association_class_primary_key_name = [:foreign_key] || Inflector.underscore(Inflector.demodulize(association_class_name)) + "_id" association_accessor_methods(association_name, association_class_name, association_class_primary_key_name, , BelongsToAssociation) module_eval do before_save <<-EOF association = instance_variable_get("@#{association_name}") if not association.nil? and association.new_record? association.save(true) self["#{association_class_primary_key_name}"] = association.id association.send(:construct_sql) end EOF end if [:counter_cache] module_eval( "after_create '#{association_class_name}.increment_counter(\"#{Inflector.pluralize(self.to_s.downcase). + "_count"}\", #{association_class_primary_key_name})" + " unless #{association_name}.nil?'" ) module_eval( "before_destroy '#{association_class_name}.decrement_counter(\"#{Inflector.pluralize(self.to_s.downcase) + "_count"}\", #{association_class_primary_key_name})" + " unless #{association_name}.nil?'" ) end # deprecated api deprecated_has_association_method(association_name) deprecated_association_comparison_method(association_name, association_class_name) 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_association_comparison_method(association_name, association_class_name) ⇒ Object
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# File 'lib/active_record/deprecated_associations.rb', line 69 def deprecated_association_comparison_method(association_name, association_class_name) module_eval <<-"end_eval", __FILE__, __LINE__ def #{association_name}?(comparison_object, force_reload = false) if comparison_object.kind_of?(#{association_class_name}) #{association_name}(force_reload) == comparison_object else raise "Comparison object is a #{association_class_name}, should have been \#{comparison_object.class.name}" end end end_eval end |
#deprecated_build_method(method_prefix, collection_name, collection_class_name, class_primary_key_name) ⇒ Object
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# File 'lib/active_record/deprecated_associations.rb', line 89 def deprecated_build_method(method_prefix, collection_name, collection_class_name, class_primary_key_name) module_eval <<-"end_eval", __FILE__, __LINE__ def #{method_prefix + collection_name}(attributes = {}) association = #{collection_class_name}.new association.attributes = attributes.merge({ "#{class_primary_key_name}" => id}) association end end_eval end |
#deprecated_collection_build_method(collection_name) ⇒ Object
:nodoc:
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# File 'lib/active_record/deprecated_associations.rb', line 61 def deprecated_collection_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_collection_create_method(collection_name) ⇒ Object
:nodoc:
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# File 'lib/active_record/deprecated_associations.rb', line 53 def deprecated_collection_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_create_method(method_prefix, collection_name, collection_class_name, class_primary_key_name) ⇒ Object
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# File 'lib/active_record/deprecated_associations.rb', line 99 def deprecated_create_method(method_prefix, collection_name, collection_class_name, class_primary_key_name) module_eval <<-"end_eval", __FILE__, __LINE__ def #{method_prefix + collection_name}(attributes = nil) #{collection_class_name}.create((attributes || {}).merge({ "#{class_primary_key_name}" => id})) 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_association_method(association_name) ⇒ Object
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# File 'lib/active_record/deprecated_associations.rb', line 81 def deprecated_has_association_method(association_name) module_eval <<-"end_eval", __FILE__, __LINE__ def has_#{association_name}?(force_reload = false) !#{association_name}(force_reload).nil? 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”.
Any additional fields added to the join table will be placed as attributes when pulling records out through has_and_belongs_to_many associations. This is helpful when have information about the association itself that you want available on retrieval. Note that any fields in the join table will override matching field names in the two joined tables. As a consequence, having an “id” field in the join table usually has the undesirable result of clobbering the “id” fields in either of the other two tables.
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 is 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.push_with_attributes(object, join_attributes)
- adds one to the collection by creating an association in the join table that also holds the attributes fromjoin_attributes
(should be a hash with the column names as keys). This can be used to have additional attributes on the join, which will be injected into the associated objects when they are retrieved through the collection. (collection.concat_with_attributes is an alias 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.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 theid
and that meets the condition that it has to be associated with this object.
Example: An Developer class declares has_and_belongs_to_many :projects
, which will add:
-
Developer#projects
-
Developer#projects<<
-
Developer#projects.push_with_attributes
-
Developer#projects.delete
-
Developer#projects.clear
-
Developer#projects.empty?
-
Developer#projects.size
-
Developer#projects.find(id)
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 infered from the association name. Sohas_and_belongs_to_many :projects
will by default be linked to theProject
class, but if the real class name isSuperProject
, 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 aPerson
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 isProject
that makes a 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 a “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 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 460 def has_and_belongs_to_many(association_id, = {}) ([ :class_name, :table_name, :foreign_key, :association_foreign_key, :conditions, :join_table, :finder_sql, :delete_sql, :insert_sql, :order, :uniq ], .keys) association_name, association_class_name, association_class_primary_key_name = associate_identification(association_id, [:class_name], [:foreign_key]) require_association_class(association_class_name) [:join_table] ||= join_table_name(undecorated_table_name(self.to_s), undecorated_table_name(association_class_name)) add_multiple_associated_save_callbacks(association_name) collection_accessor_methods(association_name, association_class_name, association_class_primary_key_name, , HasAndBelongsToManyAssociation) before_destroy_sql = "DELETE FROM #{[:join_table]} WHERE #{association_class_primary_key_name} = \\\#{self.quoted_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 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 does not destroy the objects. -
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 theid
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 withattributes
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 withattributes
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 toClients.find_all "firm_id = #{id}"
) -
Firm#clients<<
-
Firm#clients.delete
-
Firm#clients.clear
-
Firm#clients.empty?
(similar tofirm.clients.size == 0
) -
Firm#clients.size
(similar toClient.count "firm_id = #{id}"
) -
Firm#clients.find
(similar toClient.find_on_conditions(id, "firm_id = #{id}")
) -
Firm#clients.find_all
(similar toClient.find_all "firm_id = #{id}"
) -
Firm#clients.build
(similar toClient.new("firm_id" => id)
) -
Firm#clients.create
(similar toc = Client.new("client_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 infered from the association name. Sohas_many :products
will by default be linked to theProduct
class, but if the real class name isSpecialProduct
, 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 aPerson
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. May not be set if :exclusively_dependent is also set. -
: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. May not be set if :dependent is also set. -
: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. -
:counter_sql
- specify a complete SQL statement to fetch the size of the association. If:finder_sql
is specified but:counter_sql
,:counter_sql
will be generated by replacing SELECT … FROM with SELECT COUNT(*) FROM.
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 214 def has_many(association_id, = {}) ([ :foreign_key, :class_name, :exclusively_dependent, :dependent, :conditions, :order, :finder_sql, :counter_sql ], .keys) association_name, association_class_name, association_class_primary_key_name = associate_identification(association_id, [:class_name], [:foreign_key]) require_association_class(association_class_name) if [:dependent] and [:exclusively_dependent] raise ArgumentError, ':dependent and :exclusively_dependent are mutually exclusive options. You may specify one or the other.' # ' ruby-mode elsif [:dependent] module_eval "before_destroy '#{association_name}.each { |o| o.destroy }'" elsif [:exclusively_dependent] module_eval "before_destroy { |record| #{association_class_name}.delete_all(%(#{association_class_primary_key_name} = \#{record.quoted_id})) }" end add_multiple_associated_save_callbacks(association_name) collection_accessor_methods(association_name, association_class_name, association_class_primary_key_name, , HasManyAssociation) # 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_collection_create_method(association_name) deprecated_collection_build_method(association_name) end |
#has_one(association_id, options = {}) ⇒ Object
Adds the following methods for retrieval 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 manager.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, sets it as the foreign key, and saves the associate object. -
association.nil?
- returns true if there is no associated object. -
association.build(attributes = {})
- returns a new object of the associated type that has been instantiated withattributes
and linked to this object through a foreign key but has not yet been saved. -
association.create(attributes = {})
- returns a new object of the associated type that has been instantiated withattributes
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 toBeneficiary.find_first "account_id = #{id}"
) -
Account#beneficiary=(beneficiary)
(similar tobeneficiary.account_id = account.id; beneficiary.save
) -
Account#beneficiary.nil?
-
Account#beneficiary.build
(similar toBeneficiary.new("account_id" => id)
) -
Account#beneficiary.create
(similar tob = Beneficiary.new("account_id" => id); b.save; b
)
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 infered from the association name. Sohas_one :manager
will by default be linked to theManager
class, but if the real class name isPerson
, 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 asan "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 aPerson
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 281 def has_one(association_id, = {}) ([ :class_name, :foreign_key, :remote, :conditions, :order, :dependent, :counter_cache ], .keys) association_name, association_class_name, association_class_primary_key_name = associate_identification(association_id, [:class_name], [:foreign_key], false) require_association_class(association_class_name) module_eval do after_save <<-EOF association = instance_variable_get("@#{association_name}") unless association.nil? association["#{association_class_primary_key_name}"] = id association.save(true) association.send(:construct_sql) end EOF end association_accessor_methods(association_name, association_class_name, association_class_primary_key_name, , HasOneAssociation) module_eval "before_destroy '#{association_name}.destroy unless #{association_name}.nil?'" if [:dependent] # deprecated api deprecated_has_association_method(association_name) deprecated_build_method("build_", association_name, association_class_name, association_class_primary_key_name) deprecated_create_method("create_", association_name, association_class_name, association_class_primary_key_name) deprecated_association_comparison_method(association_name, association_class_name) end |