Documenting the :inverse_of option for associations

railties/guides/source/association_basics.textile

@@ -358,6 +358,7 @@ Here are a few things you should know to make efficient use of Active Record ass
 * Avoiding name collisions
 * Updating the schema
 * Controlling association scope
+* Bi-drectional associations
 
 h4. Controlling Caching
 
@@ -501,6 +502,59 @@ module MyApplication
 end
 </ruby>
 
+h4. Bi-directional Associations
+
+It's normal for associations to work in two directions, requiring declaration on two different models:
+
+<ruby>
+class Customer < ActiveRecord::Base
+  has_many :orders
+end
+
+class Order < ActiveRecord::Base
+  belongs_to :customer
+end
+</ruby>
+
+By default, Active Record doesn't know about the connection between these associations. This can lead to two copies of an object getting out of sync:
+
+<ruby>
+c = Customer.first
+o = c.orders.first
+c.first_name == o.customer.first_name # => true
+c.first_name = 'Manny'
+c.first_name == o.customer.first_name # => false
+</ruby>
+
+This happens because c and o.customer are two different in-memory representations of the same data, and neither one is automatically refreshed from changes to the other. Active Record provides the +:inverse_of+ option so that you can inform it of these relations:
+
+<ruby>
+class Customer < ActiveRecord::Base
+  has_many :orders, :inverse_of => :customer
+end
+
+class Order < ActiveRecord::Base
+  belongs_to :customer, :inverse_of => :orders
+end
+</ruby>
+
+With these changes, Active Record will only load one copy of the customer object, preventing inconsistencies and making your application more efficient:
+
+<ruby>
+c = Customer.first
+o = c.orders.first
+c.first_name == o.customer.first_name # => true
+c.first_name = 'Manny'
+c.first_name == o.customer.first_name # => true
+</ruby>
+
+There are a few limitations to +inverse_of+ support:
+
+* They do not work with <tt>:through</tt> associations.
+* They do not work with <tt>:polymorphic</tt> associations.
+* They do not work with <tt>:as</tt> associations.
+* For +belongs_to+ associations, +has_many+ inverse associations are ignored.
+
 h3. Detailed Association Reference
 
 The following sections give the details of each type of association, including the methods that they add and the options that you can use when declaring an association.
@@ -594,6 +648,7 @@ The +belongs_to+ association supports these options:
 * +:dependent+
 * +:foreign_key+
 * +:include+
+* +:inverse_of+
 * +:polymorphic+
 * +:readonly+
 * +:select+
@@ -720,6 +775,20 @@ end
 
 NOTE: There's no need to use +:include+ for immediate associations - that is, if you have +Order belongs_to :customer+, then the customer is eager-loaded automatically when it's needed.
 
+h6(#belongs_to-inverse_of). +:inverse_of+
+
+The +:inverse_of+ option specifies the name of the +has_many+ or +has_one+ association that is the inverse of this association. Does not work in combination with the +:polymorphic+ options.
+
+<ruby>
+class Customer < ActiveRecord::Base
+  has_many :orders, :inverse_of => :customer
+end
+
+class Order < ActiveRecord::Base
+  belongs_to :customer, :inverse_of => :orders
+end
+</ruby>
+
 h6(#belongs_to-polymorphic). +:polymorphic+
 
 Passing +true+ to the +:polymorphic+ option indicates that this is a polymorphic association. Polymorphic associations were discussed in detail <a href="#polymorphic-associations">earlier in this guide</a>.
@@ -859,6 +928,7 @@ The +has_one+ association supports these options:
 * +:dependent+
 * +:foreign_key+
 * +:include+
+* +:inverse_of+
 * +:order+
 * +:primary_key+
 * +:readonly+
@@ -948,6 +1018,20 @@ class Representative < ActiveRecord::Base
 end
 </ruby>
 
+h6(#has_one-inverse_of). +:inverse_of+
+
+The +:inverse_of+ option specifies the name of the +belongs_to+ association that is the inverse of this association. Does not work in combination with the +:through+ or +:as+ options.
+
+<ruby>
+class Supplier < ActiveRecord::Base
+  has_one :account, :inverse_of => :supplier
+end
+
+class Account < ActiveRecord::Base
+  belongs_to :supplier, :inverse_of => :account
+end
+</ruby>
+
 h6(#has_one-order). +:order+
 
 The +:order+ option dictates the order in which associated objects will be received (in the syntax used by an SQL +ORDER BY+ clause). Because a +has_one+ association will only retrieve a single associated object, this option should not be needed.
@@ -1177,6 +1261,7 @@ The +has_many+ association supports these options:
 * +:foreign_key+
 * +:group+
 * +:include+
+* +:inverse_of+
 * +:limit+
 * +:offset+
 * +:order+
@@ -1316,6 +1401,20 @@ class LineItem < ActiveRecord::Base
 end
 </ruby>
 
+h6(#has_many-inverse_of). +:inverse_of+
+
+The +:inverse_of+ option specifies the name of the +belongs_to+ association that is the inverse of this association. Does not work in combination with the +:through+ or +:as+ options.
+
+<ruby>
+class Customer < ActiveRecord::Base
+  has_many :orders, :inverse_of => :customer
+end
+
+class Order < ActiveRecord::Base
+  belongs_to :customer, :inverse_of => :orders
+end
+</ruby>
+
 h6(#has_many-limit). +:limit+
 
 The +:limit+ option lets you restrict the total number of objects that will be fetched through an association.