root/trunk/activerecord/lib/active_record/associations.rb

require 'active_record/associations/association_proxy'
require 'active_record/associations/association_collection'
require 'active_record/associations/belongs_to_association'
require 'active_record/associations/belongs_to_polymorphic_association'
require 'active_record/associations/has_one_association'
require 'active_record/associations/has_many_association'
require 'active_record/associations/has_many_through_association'
require 'active_record/associations/has_and_belongs_to_many_association'
require 'active_record/associations/has_one_through_association'

module ActiveRecord
  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 HasManyThroughAssociationPolymorphicError < 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 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 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(&:inspect).to_sentence :connector => 'or'} 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 :connector => 'or'}?")
    end
  end

  class HasManyThroughSourceAssociationMacroError < ActiveRecordError #:nodoc:
    def initialize(reflection)
      through_reflection = reflection.through_reflection
      source_reflection  = reflection.source_reflection
      super("Invalid source reflection macro :#{source_reflection.macro}#{" :through" if source_reflection.options[:through]} for has_many #{reflection.name.inspect}, :through => #{through_reflection.name.inspect}.  Use :source to specify the source reflection.")
    end
  end

  class HasManyThroughCantAssociateThroughHasManyReflection < 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 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

  module Associations # :nodoc:
    def self.included(base)
      base.extend(ClassMethods)
    end

    # Clears out the association cache
    def clear_association_cache #:nodoc:
      self.class.reflect_on_all_associations.to_a.each do |assoc|
        instance_variable_set "@#{assoc.name}", nil
      end unless self.new_record?
    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. 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:
    # * <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.find(: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 ActiveRecord::Base. Since the association
    # adds a method with that name to its model, it will override the inherited method and break things.
    # For instance, #attributes and #connection would be bad choices for association names.
    #
    # == Auto-generated methods
    #
    # === Singular associations (one-to-one)
    #                                     |            |  belongs_to  |
    #   generated methods                 | belongs_to | :polymorphic | has_one
    #   ----------------------------------+------------+--------------+---------
    #   #other                            |     X      |      X       |    X
    #   #other=(other)                    |     X      |      X       |    X
    #   #build_other(attributes={})       |     X      |              |    X
    #   #create_other(attributes={})      |     X      |              |    X
    #   #other.create!(attributes={})     |            |              |    X
    #   #other.nil?                       |     X      |      X       |
    #
    # ===Collection associations (one-to-many / many-to-many)
    #                                     |       |          | has_many
    #   generated methods                 | habtm | has_many | :through
    #   ----------------------------------+-------+----------+----------
    #   #others                           |   X   |    X     |    X
    #   #others=(other,other,...)         |   X   |    X     |    X
    #   #other_ids                        |   X   |    X     |    X
    #   #other_ids=(id,id,...)            |   X   |    X     |    X
    #   #others<<                         |   X   |    X     |    X
    #   #others.push                      |   X   |    X     |    X
    #   #others.concat                    |   X   |    X     |    X
    #   #others.build(attributes={})      |   X   |    X     |    X
    #   #others.create(attributes={})     |   X   |    X     |    X
    #   #others.create!(attributes={})    |   X   |    X     |    X
    #   #others.size                      |   X   |    X     |    X
    #   #others.length                    |   X   |    X     |    X
    #   #others.count                     |   X   |    X     |    X
    #   #others.sum(args*,&block)         |   X   |    X     |    X
    #   #others.empty?                    |   X   |    X     |    X
    #   #others.clear                     |   X   |    X     |    X
    #   #others.delete(other,other,...)   |   X   |    X     |    X
    #   #others.delete_all                |   X   |    X     |
    #   #others.destroy_all               |   X   |    X     |    X
    #   #others.find(*args)               |   X   |    X     |    X
    #   #others.find_first                |   X   |          |
    #   #others.uniq                      |   X   |    X     |    X
    #   #others.reset                     |   X   |    X     |    X
    #
    # == Cardinality and associations
    #
    # ActiveRecord 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. Example:
    #
    #   class User < ActiveRecord::Base
    #     # I reference an account.
    #     belongs_to :account
    #   end
    #
    #   class Account < ActiveRecord::Base
    #     # One user references me.
    #     has_one :user
    #   end
    #
    # The tables for these classes could look something like:
    #
    #   CREATE TABLE users (
    #     id int(11) NOT NULL auto_increment,
    #     account_id int(11) default NULL,
    #     name varchar default NULL,
    #     PRIMARY KEY  (id)
    #   )
    #
    #   CREATE TABLE accounts (
    #     id int(11) NOT NULL auto_increment,
    #     name varchar default NULL,
    #     PRIMARY KEY  (id)
    #   )
    #
    # == Unsaved objects and associations
    #
    # You can manipulate objects and associations before they are saved to the database, but there is some special behavior you should be
    # aware of, mostly involving the saving of associated objects.
    #
    # === One-to-one associations
    #
    # * Assigning an object to a +has_one+ association automatically saves that object and the object being replaced (if there is one), in
    #   order to update their primary keys - except if the parent object is unsaved (<tt>new_record? == true</tt>).
    # * 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 <tt>#association.build</tt> 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 <tt>#push</tt> or similar) fails, then <tt>#push</tt> returns +false+.
    # * 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 lifecycle of an Active Record object, you can also define callbacks that get
    # triggered when you add an object to or remove an object from an association collection. Example:
    #
    #   class Project
    #     has_and_belongs_to_many :developers, :after_add => :evaluate_velocity
    #
    #     def evaluate_velocity(developer)
    #       ...
    #     end
    #   end
    #
    # It's possible to stack callbacks by passing them as an array. Example:
    #
    #   class Project
    #     has_and_belongs_to_many :developers, :after_add => [:evaluate_velocity, Proc.new { |p, d| p.shipping_date = Time.now}]
    #   end
    #
    # Possible callbacks are: +before_add+, +after_add+, +before_remove+ and +after_remove+.
    #
    # Should any of the +before_add+ callbacks throw an exception, the object does not get added to the collection. Same with
    # the +before_remove+ callbacks; if an exception is thrown the object doesn't get removed.
    #
    # === Association extensions
    #
    # The proxy objects that control the access to associations can be extended through anonymous modules. This is especially
    # beneficial for adding new finders, creators, and other factory-type methods that are only used as part of this association.
    # Example:
    #
    #   class Account < ActiveRecord::Base
    #     has_many :people do
    #       def find_or_create_by_name(name)
    #         first_name, last_name = name.split(" ", 2)
    #         find_or_create_by_first_name_and_last_name(first_name, last_name)
    #       end
    #     end
    #   end
    #
    #   person = Account.find(:first).people.find_or_create_by_name("David Heinemeier Hansson")
    #   person.first_name # => "David"
    #   person.last_name  # => "Heinemeier Hansson"
    #
    # If you need to share the same extensions between many associations, you can use a named extension module. Example:
    #
    #   module FindOrCreateByNameExtension
    #     def find_or_create_by_name(name)
    #       first_name, last_name = name.split(" ", 2)
    #       find_or_create_by_first_name_and_last_name(first_name, last_name)
    #     end
    #   end
    #
    #   class Account < ActiveRecord::Base
    #     has_many :people, :extend => FindOrCreateByNameExtension
    #   end
    #
    #   class Company < ActiveRecord::Base
    #     has_many :people, :extend => FindOrCreateByNameExtension
    #   end
    #
    # If you need to use multiple named extension modules, you can specify an array of modules with the <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. Example:
    #
    #   class Account < ActiveRecord::Base
    #     has_many :people, :extend => [FindOrCreateByNameExtension, FindRecentExtension]
    #   end
    #
    # Some extensions can only be made to work with knowledge of the association proxy's internals.
    # Extensions can access relevant state using accessors on the association proxy:
    #
    # * +proxy_owner+ - Returns the object the association is part of.
    # * +proxy_reflection+ - Returns the reflection object that describes the association.
    # * +proxy_target+ - Returns the associated object for +belongs_to+ and +has_one+, or the collection of associated objects for +has_many+ and +has_and_belongs_to_many+.
    #
    # === Association Join Models
    #
    # Has Many associations can be configured with the <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.find :first
    #   @author.authorships.collect { |a| a.book } # selects all books that the author's authorships belong to.
    #   @author.books                              # selects all books by using the Authorship join model
    #
    # You can also go through a +has_many+ association on the join model:
    #
    #   class Firm < ActiveRecord::Base
    #     has_many   :clients
    #     has_many   :invoices, :through => :clients
    #   end
    #
    #   class Client < ActiveRecord::Base
    #     belongs_to :firm
    #     has_many   :invoices
    #   end
    #
    #   class Invoice < ActiveRecord::Base
    #     belongs_to :client
    #   end
    #
    #   @firm = Firm.find :first
    #   @firm.clients.collect { |c| c.invoices }.flatten # select all invoices for all clients of the firm
    #   @firm.invoices                                   # selects all invoices by going through the Client join model.
    #
    # === Polymorphic Associations
    #
    # Polymorphic associations on models are not restricted on what types of models they can be associated with.  Rather, they
    # specify an interface that a +has_many+ association must adhere to.
    #
    #   class Asset < ActiveRecord::Base
    #     belongs_to :attachable, :polymorphic => true
    #   end
    #
    #   class Post < ActiveRecord::Base
    #     has_many :assets, :as => :attachable         # The :as option specifies the polymorphic interface to use.
    #   end
    #
    #   @asset.attachable = @post
    #
    # This works by using a type column in addition to a foreign key to specify the associated record.  In the Asset example, you'd need
    # an +attachable_id+ integer column and an +attachable_type+ string column.
    #
    # Using polymorphic associations in combination with single table inheritance (STI) is a little tricky. In order
    # for the associations to work as expected, ensure that you store the base model for the STI models in the
    # type column of the polymorphic association. To continue with the asset example above, suppose there are guest posts
    # and member posts that use the posts table for STI. In this case, there must be a +type+ column in the posts table.
    #
    #   class Asset < ActiveRecord::Base
    #     belongs_to :attachable, :polymorphic => true
    #
    #     def attachable_type=(sType)
    #        super(sType.to_s.classify.constantize.base_class.to_s)
    #     end
    #   end
    #
    #   class Post < ActiveRecord::Base
    #     # because we store "Post" in attachable_type now :dependent => :destroy will work
    #     has_many :assets, :as => :attachable, :dependent => :destroy
    #   end
    #
    #   class GuestPost < Post
    #   end
    #
    #   class MemberPost < Post
    #   end
    #
    # == Caching
    #
    # All of the methods are built on a simple caching principle that will keep the result of the last query around unless specifically
    # instructed not to. The cache is even shared across methods to make it even cheaper to use the macro-added methods without
    # worrying too much about performance at the first go. Example:
    #
    #   project.milestones             # fetches milestones from the database
    #   project.milestones.size        # uses the milestone cache
    #   project.milestones.empty?      # uses the milestone cache
    #   project.milestones(true).size  # fetches milestones from the database
    #   project.milestones             # uses the milestone cache
    #
    # == Eager loading of associations
    #
    # Eager loading is a way to find objects of a certain class and a number of named associations along with it in a single SQL call. This is
    # one of the easiest ways of to prevent the dreaded 1+N problem in which fetching 100 posts that each need to display their author
    # triggers 101 database queries. Through the use of eager loading, the 101 queries can be reduced to 1. Example:
    #
    #   class Post < ActiveRecord::Base
    #     belongs_to :author
    #     has_many   :comments
    #   end
    #
    # Consider the following loop using the class above:
    #
    #   for post in Post.find(:all)
    #     puts "Post:            " + post.title
    #     puts "Written by:      " + post.author.name
    #     puts "Last comment on: " + post.comments.first.created_on
    #   end
    #
    # To iterate over these one hundred posts, we'll generate 201 database queries. Let's first just optimize it for retrieving the author:
    #
    #   for post in Post.find(:all, :include => :author)
    #
    # This references the name of the +belongs_to+ association that also used the <tt>:author</tt> symbol, so the find will now weave in a join something
    # like this: <tt>LEFT OUTER JOIN authors ON authors.id = posts.author_id</tt>. Doing so will cut down the number of queries from 201 to 101.
    #
    # We can improve upon the situation further by referencing both associations in the finder with:
    #
    #   for post in Post.find(:all, :include => [ :author, :comments ])
    #
    # That'll add another join along the lines of: <tt>LEFT OUTER JOIN comments ON comments.post_id = posts.id</tt>. And we'll be down to 1 query.
    #
    # To include a deep hierarchy of associations, use a hash:
    #
    #   for post in Post.find(:all, :include => [ :author, { :comments => { :author => :gravatar } } ])
    #
    # That'll grab not only all the comments but all their authors and gravatar pictures.  You can mix and match
    # symbols, arrays and hashes in any combination to describe the associations you want to load.
    #
    # All of this power shouldn't fool you into thinking that you can pull out huge amounts of data with no performance penalty just because you've reduced
    # the number of queries. The database still needs to send all the data to Active Record and it still needs to be processed. So it's no
    # catch-all for performance problems, but it's a great way to cut down on the number of queries in a situation as the one described above.
    #
    # Since the eager loading pulls from multiple tables, you'll have to disambiguate any column references in both conditions and orders. So
    # <tt>:order => "posts.id DESC"</tt> will work while <tt>:order => "id DESC"</tt> will not. Because eager loading generates the +SELECT+ statement too, the
    # <tt>:select</tt> option is ignored.
    #
    # You can use eager loading on multiple associations from the same table, but you cannot use those associations in orders and conditions
    # as there is currently not any way to disambiguate them. Eager loading will not pull additional attributes on join tables, so "rich
    # associations" with +has_and_belongs_to_many+ are not a good fit for eager loading.
    #
    # When eager loaded, conditions are interpolated in the context of the model class, not the model instance.  Conditions are lazily interpolated
    # before the actual model exists.
    #
    # Eager loading is not supported with polymorphic associations up to (and including)
    # version 2.0.2. Given
    #
    #   class Address < ActiveRecord::Base
    #     belongs_to :addressable, :polymorphic => true
    #   end
    #
    # a call that tries to eager load the addressable model
    #
    #   Address.find(:all, :include => :addressable)   # INVALID
    #
    # will raise <tt>ActiveRecord::EagerLoadPolymorphicError</tt>. 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 early query.
    #
    # In versions greater than 2.0.2 eager loading in polymorphic associations is supported
    # thanks to a change in the overall preloading strategy.
    #
    # It does work the other way around though: if the <tt>User</tt> model is <tt>addressable</tt> you can eager load
    # their addresses with <tt>:include</tt> just fine, every piece needed to construct the query is known beforehand.
    #
    # == Table Aliasing
    #
    # ActiveRecord uses table aliasing in the case that a table is referenced multiple times in a join.  If a table is referenced only once,
    # the standard table name is used.  The second time, the table is aliased as <tt>#{reflection_name}_#{parent_table_name}</tt>.  Indexes are appended
    # for any more successive uses of the table name.
    #
    #   Post.find :all, :include => :comments
    #   # => SELECT ... FROM posts LEFT OUTER JOIN comments ON ...
    #   Post.find :all, :include => :special_comments # STI
    #   # => SELECT ... FROM posts LEFT OUTER JOIN comments ON ... AND comments.type = 'SpecialComment'
    #   Post.find :all, :include => [:comments, :special_comments] # special_comments is the reflection name, posts is the parent table name
    #   # => SELECT ... FROM posts LEFT OUTER JOIN comments ON ... LEFT OUTER JOIN comments special_comments_posts
    #
    # Acts as tree example:
    #
    #   TreeMixin.find :all, :include => :children
    #   # => SELECT ... FROM mixins LEFT OUTER JOIN mixins childrens_mixins ...
    #   TreeMixin.find :all, :include => {:children => :parent} # using cascading eager includes
    #   # => SELECT ... FROM mixins LEFT OUTER JOIN mixins childrens_mixins ...
    #                               LEFT OUTER JOIN parents_mixins ...
    #   TreeMixin.find :all, :include => {:children => {:parent => :children}}
    #   # => SELECT ... FROM mixins LEFT OUTER JOIN mixins childrens_mixins ...
    #                               LEFT OUTER JOIN parents_mixins ...
    #                               LEFT OUTER JOIN mixins childrens_mixins_2
    #
    # Has and Belongs to Many join tables use the same idea, but add a <tt>_join</tt> suffix:
    #
    #   Post.find :all, :include => :categories
    #   # => SELECT ... FROM posts LEFT OUTER JOIN categories_posts ... LEFT OUTER JOIN categories ...
    #   Post.find :all, :include => {:categories => :posts}
    #   # => SELECT ... FROM posts LEFT OUTER JOIN categories_posts ... LEFT OUTER JOIN categories ...
    #                              LEFT OUTER JOIN categories_posts posts_categories_join LEFT OUTER JOIN posts posts_categories
    #   Post.find :all, :include => {:categories => {:posts => :categories}}
    #   # => SELECT ... FROM posts LEFT OUTER JOIN categories_posts ... LEFT OUTER JOIN categories ...
    #                              LEFT OUTER JOIN categories_posts posts_categories_join LEFT OUTER JOIN posts posts_categories
    #                              LEFT OUTER JOIN categories_posts categories_posts_join LEFT OUTER JOIN categories categories_posts
    #
    # If you wish to specify your own custom joins using a <tt>:joins</tt> option, those table names will take precedence over the eager associations:
    #
    #   Post.find :all, :include => :comments, :joins => "inner join comments ..."
    #   # => SELECT ... FROM posts LEFT OUTER JOIN comments_posts ON ... INNER JOIN comments ...
    #   Post.find :all, :include => [:comments, :special_comments], :joins => "inner join comments ..."
    #   # => SELECT ... FROM posts LEFT OUTER JOIN comments comments_posts ON ...
    #                              LEFT OUTER JOIN comments special_comments_posts ...
    #                              INNER JOIN comments ...
    #
    # Table aliases are automatically truncated according to the maximum length of table identifiers according to the specific database.
    #
    # == Modules
    #
    # By default, associations will look for objects within the current module scope. Consider:
    #
    #   module MyApplication
    #     module Business
    #       class Firm < ActiveRecord::Base
    #          has_many :clients
    #        end
    #
    #       class Company < ActiveRecord::Base; end
    #     end
    #   end
    #
    # When <tt>Firm#clients</tt> is called, it will in turn call <tt>MyApplication::Business::Company.find(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.  Example:
    #
    #   module MyApplication
    #     module Business
    #       class Firm < ActiveRecord::Base; end
    #     end
    #
    #     module Billing
    #       class Account < ActiveRecord::Base
    #         belongs_to :firm, :class_name => "MyApplication::Business::Firm"
    #       end
    #     end
    #   end
    #
    # == Type safety with <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
      # 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
      # <tt>has_many :clients</tt> would add among others <tt>clients.empty?</tt>.
      # * <tt>collection(force_reload = false)</tt> - returns an array of all the associated objects.
      #   An empty array is returned if none are found.
      # * <tt>collection<<(object, ...)</tt> - adds one or more objects to the collection by setting their foreign keys to the collection's primary key.
      # * <tt>collection.delete(object, ...)</tt> - removes one or more objects from the collection by setting their foreign keys to NULL.
      #   This will also destroy the objects if they're declared as +belongs_to+ and dependent on this model.
      # * <tt>collection=objects</tt> - replaces the collections content by deleting and adding objects as appropriate.
      # * <tt>collection_singular_ids</tt> - returns an array of the associated objects' ids
      # * <tt>collection_singular_ids=ids</tt> - replace the collection with the objects identified by the primary keys in +ids+
      # * <tt>collection.clear</tt> - 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 database if <tt>:dependent => :delete_all</tt>,
      #   otherwise sets their foreign keys to NULL.
      # * <tt>collection.empty?</tt> - returns +true+ if there are no associated objects.
      # * <tt>collection.size</tt> - returns the number of associated objects.
      # * <tt>collection.find</tt> - finds an associated object according to the same rules as Base.find.
      # * <tt>collection.build(attributes = {}, ...)</tt> - returns one or more new objects of the collection type that have been instantiated
      #   with +attributes+ and linked to this object through a foreign key, but have not yet been saved. *Note:* This only works if an
      #   associated object already exists, not if it's +nil+!
      # * <tt>collection.create(attributes = {})</tt> - returns a new object of the collection type that has been instantiated
      #   with +attributes+, linked to this object through a foreign key, and that has already been saved (if it passed the validation).
      #   *Note:* This only works if an associated object already exists, not if it's +nil+!
      #
      # Example: A +Firm+ class declares <tt>has_many :clients</tt>, which will add:
      # * <tt>Firm#clients</tt> (similar to <tt>Clients.find :all, :conditions => "firm_id = #{id}"</tt>)
      # * <tt>Firm#clients<<</tt>
      # * <tt>Firm#clients.delete</tt>
      # * <tt>Firm#clients=</tt>
      # * <tt>Firm#client_ids</tt>
      # * <tt>Firm#client_ids=</tt>
      # * <tt>Firm#clients.clear</tt>
      # * <tt>Firm#clients.empty?</tt> (similar to <tt>firm.clients.size == 0</tt>)
      # * <tt>Firm#clients.size</tt> (similar to <tt>Client.count "firm_id = #{id}"</tt>)
      # * <tt>Firm#clients.find</tt> (similar to <tt>Client.find(id, :conditions => "firm_id = #{id}")</tt>)
      # * <tt>Firm#clients.build</tt> (similar to <tt>Client.new("firm_id" => id)</tt>)
      # * <tt>Firm#clients.create</tt> (similar to <tt>c = Client.new("firm_id" => id); c.save; c</tt>)
      # The declaration can also include an options hash to specialize the behavior of the association.
      #
      # Options are:
      # * <tt>:class_name</tt>  - specify the class name of the association. Use it only if that name can't be inferred
      #   from the association name. So <tt>has_many :products</tt> 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.
      # * <tt>:conditions</tt>  - specify the conditions that the associated objects must meet in order to be included as a +WHERE+
      #   SQL fragment, such as <tt>price > 5 AND name LIKE 'B%'</tt>.  Record creations from the association are scoped if a hash
      #   is used.  <tt>has_many :posts, :conditions => {:published => true}</tt> will create published posts with <tt>@blog.posts.create</tt>
      #   or <tt>@blog.posts.build</tt>.
      # * <tt>:order</tt>       - specify the order in which the associated objects are returned as an <tt>ORDER BY</tt> SQL fragment,
      #   such as <tt>last_name, first_name DESC</tt>
      # * <tt>:foreign_key</tt> - 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+.
      # * <tt>:dependent</tt>   - if set to <tt>:destroy</tt> all the associated objects are destroyed
      #   alongside this object by calling their destroy method.  If set to <tt>:delete_all</tt> all associated
      #   objects are deleted *without* calling their destroy method.  If set to <tt>:nullify</tt> all associated
      #   objects' foreign keys are set to +NULL+ *without* calling their save callbacks. *Warning:* This option is ignored when also using
      #   the <tt>through</tt> option.
      # * <tt>:finder_sql</tt>  - specify a complete SQL statement to fetch the association. This is a good way to go for complex
      #   associations that depend on multiple tables. Note: When this option is used, +find_in_collection+ is _not_ added.
      # * <tt>:counter_sql</tt>  - specify a complete SQL statement to fetch the size of the association. If <tt>:finder_sql</tt> is
      #   specified but not <tt>:counter_sql</tt>, <tt>:counter_sql</tt> will be generated by replacing <tt>SELECT ... FROM</tt> with <tt>SELECT COUNT(*) FROM</tt>.
      # * <tt>:extend</tt>  - specify a named module for extending the proxy. See "Association extensions".
      # * <tt>:include</tt>  - specify second-order associations that should be eager loaded when the collection is loaded.
      # * <tt>:group</tt>: An attribute name by which the result should be grouped. Uses the <tt>GROUP BY</tt> SQL-clause.
      # * <tt>:limit</tt>: An integer determining the limit on the number of rows that should be returned.
      # * <tt>:offset</tt>: An integer determining the offset from where the rows should be fetched. So at 5, it would skip the first 4 rows.
      # * <tt>:select</tt>: By default, this is <tt>*</tt> as in <tt>SELECT * FROM</tt>, but can be changed if you, for example, want to do a join
      #   but not include the joined columns.
      # * <tt>:as</tt>: Specifies a polymorphic interface (See <tt>#belongs_to</tt>).
      # * <tt>:through</tt>: Specifies a Join Model through which to perform the query.  Options for <tt>:class_name</tt> and <tt>:foreign_key</tt>
      #   are ignored, as the association uses the source reflection. You can only use a <tt>:through</tt> query through a <tt>belongs_to</tt>
      #   or <tt>has_many</tt> association on the join model.
      # * <tt>:source</tt>: Specifies the source association name used by <tt>has_many :through</tt> queries.  Only use it if the name cannot be
      #   inferred from the association.  <tt>has_many :subscribers, :through => :subscriptions</tt> will look for either <tt>:subscribers</tt> or