associations.rb | searchcode

/activerecord/lib/active_record/associations.rb

https://github.com/ghar/rails
Ruby | 1012 lines | 135 code | 29 blank | 848 comment | 4 complexity | 9c652617bb4669bc6f6257f73a32ce76 MD5 | raw file

require 'active_support/core_ext/array/wrap'
require 'active_support/core_ext/enumerable'
require 'active_support/core_ext/module/delegation'
require 'active_support/core_ext/object/blank'
require 'active_support/core_ext/string/conversions'
require 'active_support/core_ext/module/remove_method'
require 'active_support/core_ext/class/attribute'

module ActiveRecord
  class InverseOfAssociationNotFoundError < ActiveRecordError #:nodoc:
    def initialize(reflection, associated_class = nil)
      super("Could not find the inverse association for #{reflection.name} (#{reflection.options[:inverse_of].inspect} in #{associated_class.nil? ? reflection.class_name : associated_class.name})")
    end
  end

  class HasManyThroughAssociationNotFoundError < ActiveRecordError #:nodoc:
    def initialize(owner_class_name, reflection)
      super("Could not find the association #{reflection.options[:through].inspect} in model #{owner_class_name}")
    end
  end

  class HasManyThroughAssociationPolymorphicSourceError < ActiveRecordError #:nodoc:
    def initialize(owner_class_name, reflection, source_reflection)
      super("Cannot have a has_many :through association '#{owner_class_name}##{reflection.name}' on the polymorphic object '#{source_reflection.class_name}##{source_reflection.name}'.")
    end
  end

  class HasManyThroughAssociationPolymorphicThroughError < ActiveRecordError #:nodoc:
    def initialize(owner_class_name, reflection)
      super("Cannot have a has_many :through association '#{owner_class_name}##{reflection.name}' which goes through the polymorphic association '#{owner_class_name}##{reflection.through_reflection.name}'.")
    end
  end

  class HasManyThroughAssociationPointlessSourceTypeError < ActiveRecordError #:nodoc:
    def initialize(owner_class_name, reflection, source_reflection)
      super("Cannot have a has_many :through association '#{owner_class_name}##{reflection.name}' with a :source_type option if the '#{reflection.through_reflection.class_name}##{source_reflection.name}' is not polymorphic. Try removing :source_type on your association.")
    end
  end

  class HasOneThroughCantAssociateThroughCollection < ActiveRecordError #:nodoc:
    def initialize(owner_class_name, reflection, through_reflection)
      super("Cannot have a has_one :through association '#{owner_class_name}##{reflection.name}' where the :through association '#{owner_class_name}##{through_reflection.name}' is a collection. Specify a has_one or belongs_to association in the :through option instead.")
    end
  end

  class HasManyThroughSourceAssociationNotFoundError < ActiveRecordError #:nodoc:
    def initialize(reflection)
      through_reflection      = reflection.through_reflection
      source_reflection_names = reflection.source_reflection_names
      source_associations     = reflection.through_reflection.klass.reflect_on_all_associations.collect { |a| a.name.inspect }
      super("Could not find the source association(s) #{source_reflection_names.collect{ |a| a.inspect }.to_sentence(:two_words_connector => ' or ', :last_word_connector => ', or ', :locale => :en)} in model #{through_reflection.klass}. Try 'has_many #{reflection.name.inspect}, :through => #{through_reflection.name.inspect}, :source => <name>'. Is it one of #{source_associations.to_sentence(:two_words_connector => ' or ', :last_word_connector => ', or ', :locale => :en)}?")
    end
  end

  class HasManyThroughCantAssociateThroughHasOneOrManyReflection < ActiveRecordError #:nodoc:
    def initialize(owner, reflection)
      super("Cannot modify association '#{owner.class.name}##{reflection.name}' because the source reflection class '#{reflection.source_reflection.class_name}' is associated to '#{reflection.through_reflection.class_name}' via :#{reflection.source_reflection.macro}.")
    end
  end

  class HasManyThroughCantAssociateNewRecords < ActiveRecordError #:nodoc:
    def initialize(owner, reflection)
      super("Cannot associate new records through '#{owner.class.name}##{reflection.name}' on '#{reflection.source_reflection.class_name rescue nil}##{reflection.source_reflection.name rescue nil}'. Both records must have an id in order to create the has_many :through record associating them.")
    end
  end

  class HasManyThroughCantDissociateNewRecords < ActiveRecordError #:nodoc:
    def initialize(owner, reflection)
      super("Cannot dissociate new records through '#{owner.class.name}##{reflection.name}' on '#{reflection.source_reflection.class_name rescue nil}##{reflection.source_reflection.name rescue nil}'. Both records must have an id in order to delete the has_many :through record associating them.")
    end
  end

  class HasManyThroughNestedAssociationsAreReadonly < ActiveRecordError #:nodoc
    def initialize(owner, reflection)
      super("Cannot modify association '#{owner.class.name}##{reflection.name}' because it goes through more than one other association.")
    end
  end

  class HasAndBelongsToManyAssociationForeignKeyNeeded < ActiveRecordError #:nodoc:
    def initialize(reflection)
      super("Cannot create self referential has_and_belongs_to_many association on '#{reflection.class_name rescue nil}##{reflection.name rescue nil}'. :association_foreign_key cannot be the same as the :foreign_key.")
    end
  end

  class EagerLoadPolymorphicError < ActiveRecordError #:nodoc:
    def initialize(reflection)
      super("Can not eagerly load the polymorphic association #{reflection.name.inspect}")
    end
  end

  class ReadOnlyAssociation < ActiveRecordError #:nodoc:
    def initialize(reflection)
      super("Can not add to a has_many :through association. Try adding to #{reflection.through_reflection.name.inspect}.")
    end
  end

  # This error is raised when trying to destroy a parent instance in N:1 or 1:1 associations
  # (has_many, has_one) when there is at least 1 child associated instance.
  # ex: if @project.tasks.size > 0, DeleteRestrictionError will be raised when trying to destroy @project
  class DeleteRestrictionError < ActiveRecordError #:nodoc:
    def initialize(name)
      super("Cannot delete record because of dependent #{name}")
    end
  end

  # See ActiveRecord::Associations::ClassMethods for documentation.
  module Associations # :nodoc:
    extend ActiveSupport::Concern

    # These classes will be loaded when associations are created.
    # So there is no need to eager load them.
    autoload :Association,           'active_record/associations/association'
    autoload :SingularAssociation,   'active_record/associations/singular_association'
    autoload :CollectionAssociation, 'active_record/associations/collection_association'
    autoload :CollectionProxy,       'active_record/associations/collection_proxy'

    autoload :BelongsToAssociation,            'active_record/associations/belongs_to_association'
    autoload :BelongsToPolymorphicAssociation, 'active_record/associations/belongs_to_polymorphic_association'
    autoload :HasAndBelongsToManyAssociation,  'active_record/associations/has_and_belongs_to_many_association'
    autoload :HasManyAssociation,              'active_record/associations/has_many_association'
    autoload :HasManyThroughAssociation,       'active_record/associations/has_many_through_association'
    autoload :HasOneAssociation,               'active_record/associations/has_one_association'
    autoload :HasOneThroughAssociation,        'active_record/associations/has_one_through_association'
    autoload :ThroughAssociation,              'active_record/associations/through_association'

    module Builder #:nodoc:
      autoload :Association,           'active_record/associations/builder/association'
      autoload :SingularAssociation,   'active_record/associations/builder/singular_association'
      autoload :CollectionAssociation, 'active_record/associations/builder/collection_association'

      autoload :BelongsTo,           'active_record/associations/builder/belongs_to'
      autoload :HasOne,              'active_record/associations/builder/has_one'
      autoload :HasMany,             'active_record/associations/builder/has_many'
      autoload :HasAndBelongsToMany, 'active_record/associations/builder/has_and_belongs_to_many'
    end

    autoload :Preloader,        'active_record/associations/preloader'
    autoload :JoinDependency,   'active_record/associations/join_dependency'
    autoload :AssociationScope, 'active_record/associations/association_scope'
    autoload :AliasTracker,     'active_record/associations/alias_tracker'
    autoload :JoinHelper,       'active_record/associations/join_helper'

    # Clears out the association cache.
    def clear_association_cache #:nodoc:
      @association_cache.clear if persisted?
    end

    # :nodoc:
    attr_reader :association_cache

    # Returns the association instance for the given name, instantiating it if it doesn't already exist
    def association(name) #:nodoc:
      association = association_instance_get(name)

      if association.nil?
        reflection  = self.class.reflect_on_association(name)
        association = reflection.association_class.new(self, reflection)
        association_instance_set(name, association)
      end

      association
    end

    private
      # Returns the specified association instance if it responds to :loaded?, nil otherwise.
      def association_instance_get(name)
        @association_cache[name.to_sym]
      end

      # Set the specified association instance.
      def association_instance_set(name, association)
        @association_cache[name] = association
      end

    # 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 <tt>attr*</tt>
    # methods.
    #
    #   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:
    # * <tt>Project#portfolio, Project#portfolio=(portfolio), Project#portfolio.nil?</tt>
    # * <tt>Project#project_manager, Project#project_manager=(project_manager), Project#project_manager.nil?,</tt>
    # * <tt>Project#milestones.empty?, Project#milestones.size, Project#milestones, Project#milestones<<(milestone),</tt>
    #   <tt>Project#milestones.delete(milestone), Project#milestones.find(milestone_id), Project#milestones.all(options),</tt>
    #   <tt>Project#milestones.build, Project#milestones.create</tt>
    # * <tt>Project#categories.empty?, Project#categories.size, Project#categories, Project#categories<<(category1),</tt>
    #   <tt>Project#categories.delete(category1)</tt>
    #
    # === A word of warning
    #
    # Don't create associations that have the same name as instance methods of
    # <tt>ActiveRecord::Base</tt>. 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
    #   create_other!(attributes={})      |     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     |    X
    #   other_ids                         |   X   |    X     |    X
    #   other_ids=(id,id,...)             |   X   |    X     |    X
    #   others<<                          |   X   |    X     |    X
    #   others.push                       |   X   |    X     |    X
    #   others.concat                     |   X   |    X     |    X
    #   others.build(attributes={})       |   X   |    X     |    X
    #   others.create(attributes={})      |   X   |    X     |    X
    #   others.create!(attributes={})     |   X   |    X     |    X
    #   others.size                       |   X   |    X     |    X
    #   others.length                     |   X   |    X     |    X
    #   others.count                      |   X   |    X     |    X
    #   others.sum(args*,&block)          |   X   |    X     |    X
    #   others.empty?                     |   X   |    X     |    X
    #   others.clear                      |   X   |    X     |    X
    #   others.delete(other,other,...)    |   X   |    X     |    X
    #   others.delete_all                 |   X   |    X     |    X
    #   others.destroy_all                |   X   |    X     |    X
    #   others.find(*args)                |   X   |    X     |    X
    #   others.exists?                    |   X   |    X     |    X
    #   others.uniq                       |   X   |    X     |    X
    #   others.reset                      |   X   |    X     |    X
    #
    # == Cardinality and associations
    #
    # Active Record associations can be used to describe one-to-one, one-to-many and many-to-many
    # relationships between models. Each model uses an association to describe its role in
    # the relation. The +belongs_to+ association is always used in the model that has
    # the foreign key.
    #
    # === One-to-one
    #
    # Use +has_one+ in the base, and +belongs_to+ in the associated model.
    #
    #   class Employee < ActiveRecord::Base
    #     has_one :office
    #   end
    #   class Office < ActiveRecord::Base
    #     belongs_to :employee    # foreign key - employee_id
    #   end
    #
    # === One-to-many
    #
    # Use +has_many+ in the base, and +belongs_to+ in the associated model.
    #
    #   class Manager < ActiveRecord::Base
    #     has_many :employees
    #   end
    #   class Employee < ActiveRecord::Base
    #     belongs_to :manager     # foreign key - manager_id
    #   end
    #
    # === Many-to-many
    #
    # There are two ways to build a many-to-many relationship.
    #
    # The first way uses a +has_many+ association with the <tt>:through</tt> 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 <tt>has_many :through</tt>. 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.
    #
    #   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.
    #
    # You can set the :autosave option on a <tt>has_one</tt>, <tt>belongs_to</tt>,
    # <tt>has_many</tt>, or <tt>has_and_belongs_to_many</tt> association. Setting it
    # to +true+ will _always_ save the members, whereas setting it to +false+ will
    # _never_ save the members. More details about :autosave option is available at
    # autosave_association.rb .
    #
    # === 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 foreign
    #   keys - except if the parent object is unsaved (<tt>new_record? == true</tt>).
    # * If either of these saves fail (due to one of the objects being invalid), an
    #   <tt>ActiveRecord::RecordNotSaved</tt> exception is raised and the assignment is
    #   cancelled.
    # * If you wish to assign an object to a +has_one+ association without saving it,
    #   use the <tt>build_association</tt> method (documented below). The object being
    #   replaced will still be saved to update its foreign key.
    # * 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 <tt>push</tt> or similar)
    #   fails, then <tt>push</tt> returns +false+.
    # * If saving fails while replacing the collection (via <tt>association=</tt>), an
    #   <tt>ActiveRecord::RecordNotSaved</tt> exception is raised and the assignment is
    #   cancelled.
    # * You can add an object to a collection without automatically saving it by using the
    #   <tt>collection.build</tt> method (documented below).
    # * All unsaved (<tt>new_record? == true</tt>) members of the collection are automatically
    #   saved when the parent is saved.
    #
    # === Association callbacks
    #
    # Similar to the normal callbacks that hook into the life cycle 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.
    #
    #   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.
    #
    #   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.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.
    #
    #   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 <tt>:extend</tt> 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.
    #
    #   class Account < ActiveRecord::Base
    #     has_many :people, :extend => [FindOrCreateByNameExtension, FindRecentExtension]
    #   end
    #
    # Some extensions can only be made to work with knowledge of the association's internals.
    # Extensions can access relevant state using the following methods (where +items+ is the
    # name of the association):
    #
    # * <tt>record.association(:items).owner</tt> - Returns the object the association is part of.
    # * <tt>record.association(:items).reflection</tt> - Returns the reflection object that describes the association.
    # * <tt>record.association(:items).target</tt> - 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+.
    #
    # However, inside the actual extension code, you will not have access to the <tt>record</tt> as
    # above. In this case, you can access <tt>proxy_association</tt>. For example,
    # <tt>record.association(:items)</tt> and <tt>record.items.proxy_association</tt> will return
    # the same object, allowing you to make calls like <tt>proxy_association.owner</tt> inside
    # association extensions.
    #
    # === Association Join Models
    #
    # Has Many associations can be configured with the <tt>:through</tt> 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.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.first
    #   @firm.clients.collect { |c| c.invoices }.flatten # select all invoices for all clients of the firm
    #   @firm.invoices                                   # selects all invoices by going through the Client join model
    #
    # Similarly you can go through a +has_one+ association on the join model:
    #
    #   class Group < ActiveRecord::Base
    #     has_many   :users
    #     has_many   :avatars, :through => :users
    #   end
    #
    #   class User < ActiveRecord::Base
    #     belongs_to :group
    #     has_one    :avatar
    #   end
    #
    #   class Avatar < ActiveRecord::Base
    #     belongs_to :user
    #   end
    #
    #   @group = Group.first
    #   @group.users.collect { |u| u.avatar }.flatten # select all avatars for all users in the group
    #   @group.avatars                                # selects all avatars by going through the User join model.
    #
    # An important caveat with going through +has_one+ or +has_many+ associations on the
    # join model is that these associations are *read-only*. For example, the following
    # would not work following the previous example:
    #
    #   @group.avatars << Avatar.new   # this would work if User belonged_to Avatar rather than the other way around
    #   @group.avatars.delete(@group.avatars.last)  # so would this
    #
    # If you are using a +belongs_to+ on the join model, it is a good idea to set the
    # <tt>:inverse_of</tt> option on the +belongs_to+, which will mean that the following example
    # works correctly (where <tt>tags</tt> is a +has_many+ <tt>:through</tt> association):
    #
    #   @post = Post.first
    #   @tag = @post.tags.build :name => "ruby"
    #   @tag.save
    #
    # The last line ought to save the through record (a <tt>Taggable</tt>). This will only work if the
    # <tt>:inverse_of</tt> is set:
    #
    #   class Taggable < ActiveRecord::Base
    #     belongs_to :post
    #     belongs_to :tag, :inverse_of => :taggings
    #   end
    #
    # === Nested Associations
    #
    # You can actually specify *any* association with the <tt>:through</tt> option, including an
    # association which has a <tt>:through</tt> option itself. For example:
    #
    #   class Author < ActiveRecord::Base
    #     has_many :posts
    #     has_many :comments, :through => :posts
    #     has_many :commenters, :through => :comments
    #   end
    #
    #   class Post < ActiveRecord::Base
    #     has_many :comments
    #   end
    #
    #   class Comment < ActiveRecord::Base
    #     belongs_to :commenter
    #   end
    #
    #   @author = Author.first
    #   @author.commenters # => People who commented on posts written by the author
    #
    # An equivalent way of setting up this association this would be:
    #
    #   class Author < ActiveRecord::Base
    #     has_many :posts
    #     has_many :commenters, :through => :posts
    #   end
    #
    #   class Post < ActiveRecord::Base
    #     has_many :comments
    #     has_many :commenters, :through => :comments
    #   end
    #
    #   class Comment < ActiveRecord::Base
    #     belongs_to :commenter
    #   end
    #
    # When using nested association, you will not be able to modify the association because there
    # is not enough information to know what modification to make. For example, if you tried to
    # add a <tt>Commenter</tt> in the example above, there would be no way to tell how to set up the
    # intermediate <tt>Post</tt> and <tt>Comment</tt> objects.
    #
    # === 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.
    #
    #   project.milestones             # fetches milestones from the database
    #   project.milestones.size        # uses the milestone cache
    #   project.milestones.empty?      # uses the milestone cache
    #   project.milestones(true).size  # fetches milestones from the database
    #   project.milestones             # uses the milestone cache
    #
    # == Eager loading of associations
    #
    # Eager loading is a way to find objects of a certain class and a number of named associations.
    # This is one of the easiest ways of to prevent the dreaded 1+N problem in which fetching 100
    # posts that each need to display their author triggers 101 database queries. Through the
    # use of eager loading, the 101 queries can be reduced to 2.
    #
    #   class Post < ActiveRecord::Base
    #     belongs_to :author
    #     has_many   :comments
    #   end
    #
    # Consider the following loop using the class above:
    #
    #   Post.all.each do |post|
    #     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:
    #
    #   Post.includes(:author).each do |post|
    #
    # This references the name of the +belongs_to+ association that also used the <tt>:author</tt>
    # symbol. After loading the posts, find will collect the +author_id+ from each one and load
    # all the referenced authors with one query. Doing so will cut down the number of queries
    # from 201 to 102.
    #
    # We can improve upon the situation further by referencing both associations in the finder with:
    #
    #   Post.includes(:author, :comments).each do |post|
    #
    # This will load all comments with a single query. This reduces the total number of queries
    # to 3. More generally the number of queries will be 1 plus the number of associations
    # named (except if some of the associations are polymorphic +belongs_to+ - see below).
    #
    # To include a deep hierarchy of associations, use a hash:
    #
    #   Post.includes(:author, {:comments => {:author => :gravatar}}).each do |post|
    #
    # That'll grab not only all the comments but all their authors and gravatar pictures.
    # You can mix and match symbols, arrays and hashes in any combination to describe the
    # associations you want to load.
    #
    # All of this power shouldn't fool you into thinking that you can pull out huge amounts
    # of data with no performance penalty just because you've reduced the number of queries.
    # The database still needs to send all the data to Active Record and it still needs to
    # be processed. So it's no catch-all for performance problems, but it's a great way to
    # cut down on the number of queries in a situation as the one described above.
    #
    # Since only one table is loaded at a time, conditions or orders cannot reference tables
    # other than the main one. If this is the case Active Record falls back to the previously
    # used LEFT OUTER JOIN based strategy. For example
    #
    #   Post.includes([:author, :comments]).where(['comments.approved = ?', true]).all
    #
    # This will result in a single SQL query with joins along the lines of:
    # <tt>LEFT OUTER JOIN comments ON comments.post_id = posts.id</tt> and
    # <tt>LEFT OUTER JOIN authors ON authors.id = posts.author_id</tt>. Note that using conditions
    # like this can have unintended consequences.
    # In the above example posts with no approved comments are not returned at all, because
    # the conditions apply to the SQL statement as a whole and not just to the association.
    # You must disambiguate column references for this fallback to happen, for example
    # <tt>:order => "author.name DESC"</tt> will work but <tt>:order => "name DESC"</tt> will not.
    #
    # If you do want eager load only some members of an association it is usually more natural
    # to include an association which has conditions defined on it:
    #
    #   class Post < ActiveRecord::Base
    #     has_many :approved_comments, :class_name => 'Comment', :conditions => ['approved = ?', true]
    #   end
    #
    #   Post.includes(:approved_comments)
    #
    # This will load posts and eager load the +approved_comments+ association, which contains
    # only those comments that have been approved.
    #
    # If you eager load an association with a specified <tt>:limit</tt> option, it will be ignored,
    # returning all the associated objects:
    #
    #   class Picture < ActiveRecord::Base
    #     has_many :most_recent_comments, :class_name => 'Comment', :order => 'id DESC', :limit => 10
    #   end
    #
    #   Picture.includes(:most_recent_comments).first.most_recent_comments # => returns all associated comments.
    #
    # When eager loaded, conditions are interpolated in the context of the model class, not
    # the model instance. Conditions are lazily interpolated before the actual model exists.
    #
    # Eager loading is supported with polymorphic associations.
    #
    #   class Address < ActiveRecord::Base
    #     belongs_to :addressable, :polymorphic => true
    #   end
    #
    # A call that tries to eager load the addressable model
    #
    #   Address.includes(:addressable)
    #
    # This will execute one query to load the addresses and load the addressables with one
    # query per addressable type.
    # For example if all the addressables are either of class Person or Company then a total
    # of 3 queries will be executed. The list of addressable types to load is determined on
    # the back of the addresses loaded. This is not supported if Active Record has to fallback
    # to the previous implementation of eager loading and will raise ActiveRecord::EagerLoadPolymorphicError.
    # The reason is that the parent model's type is a column value so its corresponding table
    # name cannot be put in the +FROM+/+JOIN+ clauses of that query.
    #
    # == Table Aliasing
    #
    # Active Record uses table aliasing in the case that a table is referenced multiple times
    # in a join. If a table is referenced only once, the standard table name is used. The
    # second time, the table is aliased as <tt>#{reflection_name}_#{parent_table_name}</tt>.
    # Indexes are appended for any more successive uses of the table name.
    #
    #   Post.joins(:comments)
    #   # => SELECT ... FROM posts INNER JOIN comments ON ...
    #   Post.joins(:special_comments) # STI
    #   # => SELECT ... FROM posts INNER JOIN comments ON ... AND comments.type = 'SpecialComment'
    #   Post.joins(:comments, :special_comments) # special_comments is the reflection name, posts is the parent table name
    #   # => SELECT ... FROM posts INNER JOIN comments ON ... INNER JOIN comments special_comments_posts
    #
    # Acts as tree example:
    #
    #   TreeMixin.joins(:children)
    #   # => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
    #   TreeMixin.joins(:children => :parent)
    #   # => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
    #                               INNER JOIN parents_mixins ...
    #   TreeMixin.joins(:children => {:parent => :children})
    #   # => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
    #                               INNER JOIN parents_mixins ...
    #                               INNER JOIN mixins childrens_mixins_2
    #
    # Has and Belongs to Many join tables use the same idea, but add a <tt>_join</tt> suffix:
    #
    #   Post.joins(:categories)
    #   # => SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ...
    #   Post.joins(:categories => :posts)
    #   # => SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ...
    #                              INNER JOIN categories_posts posts_categories_join INNER JOIN posts posts_categories
    #   Post.joins(:categories => {:posts => :categories})
    #   # => SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ...
    #                              INNER JOIN categories_posts posts_categories_join INNER JOIN posts posts_categories
    #                              INNER JOIN categories_posts categories_posts_join INNER JOIN categories categories_posts_2
    #
    # If you wish to specify your own custom joins using <tt>joins</tt> method, those table
    # names will take precedence over the eager associations:
    #
    #   Post.joins(:comments).joins("inner join comments ...")
    #   # => SELECT ... FROM posts INNER JOIN comments_posts ON ... INNER JOIN comments ...
    #   Post.joins(:comments, :special_comments).joins("inner join comments ...")
    #   # => SELECT ... FROM posts INNER JOIN comments comments_posts ON ...
    #                              INNER JOIN comments special_comments_posts ...
    #                              INNER JOIN comments ...
    #
    # Table aliases are automatically truncated according to the maximum length of table identifiers
    # according to the specific database.
    #
    # == Modules
    #
    # By default, associations will look for objects within the current module scope. Consider:
    #
    #   module MyApplication
    #     module Business
    #       class Firm < ActiveRecord::Base
    #          has_many :clients
    #        end
    #
    #       class Client < ActiveRecord::Base; end
    #     end
    #   end
    #
    # When <tt>Firm#clients</tt> is called, it will in turn call
    # <tt>MyApplication::Business::Client.find_all_by_firm_id(firm.id)</tt>.
    # If you want to associate with a class in another module scope, this can be done by
    # specifying the complete class name.
    #
    #   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
    #
    # == Bi-directional associations
    #
    # When you specify an association there is usually an association on the associated model
    # that specifies the same relationship in reverse. For example, with the following models:
    #
    #    class Dungeon < ActiveRecord::Base
    #      has_many :traps
    #      has_one :evil_wizard
    #    end
    #
    #    class Trap < ActiveRecord::Base
    #      belongs_to :dungeon
    #    end
    #
    #    class EvilWizard < ActiveRecord::Base
    #      belongs_to :dungeon
    #    end
    #
    # The +traps+ association on +Dungeon+ and the +dungeon+ association on +Trap+ are
    # the inverse of each other and the inverse of the +dungeon+ association on +EvilWizard+
    # is the +evil_wizard+ association on +Dungeon+ (and vice-versa). By default,
    # Active Record doesn't know anything about these inverse relationships and so no object
    # loading optimization is possible. For example:
    #
    #    d = Dungeon.first
    #    t = d.traps.first
    #    d.level == t.dungeon.level # => true
    #    d.level = 10
    #    d.level == t.dungeon.level # => false
    #
    # The +Dungeon+ instances +d+ and <tt>t.dungeon</tt> in the above example refer to
    # the same object data from the database, but are actually different in-memory copies
    # of that data. Specifying the <tt>:inverse_of</tt> option on associations lets you tell
    # Active Record about inverse relationships and it will optimise object loading. For
    # example, if we changed our model definitions to:
    #
    #    class Dungeon < ActiveRecord::Base
    #      has_many :traps, :inverse_of => :dungeon
    #      has_one :evil_wizard, :inverse_of => :dungeon
    #    end
    #
    #    class Trap < ActiveRecord::Base
    #      belongs_to :dungeon, :inverse_of => :traps
    #    end
    #
    #    class EvilWizard < ActiveRecord::Base
    #      belongs_to :dungeon, :inverse_of => :evil_wizard
    #    end
    #
    # Then, from our code snippet above, +d+ and <tt>t.dungeon</tt> are actually the same
    # in-memory instance and our final <tt>d.level == t.dungeon.level</tt> will return +true+.
    #
    # There are limitations to <tt>:inverse_of</tt> support:
    #
    # * does not work with <tt>:through</tt> associations.
    # * does not work with <tt>:polymorphic</tt> associations.
    # * for +belongs_to+ associations +has_many+ inverse associations are ignored.
    #
    # == Deleting from associations
    #
    # === Dependent associations
    #
    # +has_many+, +has_one+ and +belongs_to+ associations support the <tt>:dependent</tt> option.
    # This allows you to specify that associated records should be deleted when the owner is
    # deleted.
    #
    # For example:
    #
    #     class Author
    #       has_many :posts, :dependent => :destroy
    #     end
    #     Author.find(1).destroy # => Will destroy all of the author's posts, too
    #
    # The <tt>:dependent</tt> option can have different values which specify how the deletion
    # is done. For more information, see the documentation for this option on the different
    # specific association types.
    #
    # === Delete or destroy?
    #
    # +has_many+ and +has_and_belongs_to_many+ associations have the methods <tt>destroy</tt>,
    # <tt>delete</tt>, <tt>destroy_all</tt> and <tt>delete_all</tt>.
    #
    # For +has_and_belongs_to_many+, <tt>delete</tt> and <tt>destroy</tt> are the same: they
    # cause the records in the join table to be removed.
    #
    # For +has_many+, <tt>destroy</tt> will always call the <tt>destroy</tt> method of the
    # record(s) being removed so that callbacks are run. However <tt>delete</tt> will either
    # do the deletion according to the strategy specified by the <tt>:dependent</tt> option, or
    # if no <tt>:dependent</tt> option is given, then it will follow the default strategy.
    # The default strategy is <tt>:nullify</tt> (set the foreign keys to <tt>nil</tt>), except for
    # +has_many+ <tt>:through</tt>, where the default strategy is <tt>delete_all</tt> (delete
    # the join records, without running their callbacks).
    #
    # There is also a <tt>clear</tt> method which is the same as <tt>delete_all</tt>, except that
    # it returns the association rather than the records which have been deleted.
    #
    # === What gets deleted?
    #
    # There is a potential pitfall here: +has_and_belongs_to_many+ and +has_many+ <tt>:through</tt>
    # associations have records in join tables, as well as the associated records. So when we
    # call one of these deletion methods, what exactly should be deleted?
    #
    # The answer is that it is assumed that deletion on an association is about removing the
    # <i>link</i> between the owner and the associated object(s), rather than necessarily the
    # associated objects themselves. So with +has_and_belongs_to_many+ and +has_many+
    # <tt>:through</tt>, the join records will be deleted, but the associated records won't.
    #
    # This makes sense if you think about it: if you were to call <tt>post.tags.delete(Tag.find_by_name('food'))</tt>
    # you would want the 'food' tag to be unlinked from the post, rather than for the tag itself
    # to be removed from the database.
    #
    # However, there are examples where this strategy doesn't make sense. For example, suppose
    # a person has many projects, and each project has many tasks. If we deleted one of a person's
    # tasks, we would probably not want the project to be deleted. In this scenario, the delete method
    # won't actually work: it can only be used if the association on the join model is a
    # +belongs_to+. In other situations you are expected to perform operations directly on
    # either the associated records or the <tt>:through</tt> association.
    #
    # With a regular +has_many+ there is no distinction between the "associated records"
    # and the "link", so there is only one choice for what gets deleted.
    #
    # With +has_and_belongs_to_many+ and +has_many+ <tt>:through</tt>, if you want to delete the
    # associated records themselves, you can always do something along the lines of
    # <tt>person.tasks.each(&:destroy)</tt>.
    #
    # == Type safety with <tt>ActiveRecord::AssociationTypeMismatch</tt>
    #
    # If you attempt to assign an object to an association that doesn't match the inferred
    # or specified <tt>:class_name</tt>, you'll get an <tt>ActiveRecord::AssociationTypeMismatch</tt>.
    #
    # == Options
    #
    # All of the association macros can be specialized through options. This makes cases
    # more complex than the simple and guessable ones possible.
    module ClassMethods
      # Specifies a one-to-many association. The following methods for retrieval and query of
      # collections of associated objects will be added:
      #
      # [collection(force_reload = false)]
      #   Returns an array of all the associated objects.
      #   An empty array is returned if none are found.
      # [collection<<(object, ...)]
      #   Adds one or more objects to the collection by setting their foreign keys to the collection's primary key.
      #   Note that this operation instantly fires update sql without waiting for the save or update call on the
      #   parent object.
      # [collection.delete(object, ...)]
      #   Removes one or more objects from the collection by setting their foreign keys to +NULL+.
      #   Objects will be in addition destroyed if they're associated with <tt>:dependent => :destroy</tt>,
      #   and deleted if they're associated with <tt>:dependent => :delete_all</tt>.
      #
      #   If the <tt>:through</tt> option is used, then the join records are deleted (rather than
      #   nullified) by default, but you can specify <tt>:dependent => :destroy</tt> or
      #   <tt>:dependent => :nullify</tt> to override this.
      # [collection=objects]
      #   Replaces the collections content by deleting and adding objects as appropriate. If the <tt>:through</tt>
      #   option is true callbacks in the join models are triggered except destroy callbacks, since deletion is
      #   direct.
      # [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+. This
      #   method loads the models and calls <tt>collection=</tt>. See above.
      # [collection.clear]
      #   Removes every object from the collection. This destroys the associated objects if they
      #   are associated with <tt>:dependent => :destroy</tt>, deletes them directly from the
      #   datab