Documenting the application configuration module.

Author: wangjohn

activerecord/lib/active_record/application_configuration.rb

@@ -2,6 +2,47 @@ module ActiveRecord
   module ApplicationConfiguration
     extend ActiveSupport::Concern
 
+    # The <tt>ApplicationConfiguration</tt> module allows you to control different
+    # configurations that have been set on a particular namespace or model. The
+    # <tt>ApplicationRecord</tt> class for a particular model can be accessed and used
+    # for finding configurations.
+    #
+    # To access this class, you can use the <tt>application_record</tt> method on
+    # any subclass of <tt>ActiveRecord::Base</tt>. For example:
+    #
+    #   class MyModel < ApplicationRecord
+    #   end
+    #
+    #   ActiveRecord::Base.application_record  # => ApplicationRecord
+    #   MyModel.application_record             # => ApplicationRecord
+    #
+    # Moreover, you can use namespacing of modules with the <tt>application_record</tt>
+    # method.
+    #
+    #   module MyNamespace
+    #     class ApplicationRecord < ::ApplicationRecord
+    #     end
+    #
+    #     class MyModel < ApplicationRecord
+    #     end
+    #   end
+    #
+    #   ActiveRecord::Base.application_record    # => ApplicationRecord
+    #   MyNamespace::MyModel.application_record  # => MyNamespace::ApplicationRecord
+    #
+    # You may additionally pass in a class to <tt>application_record</tt>, and
+    # it will look for the configuration of that class. For example:
+    #
+    #   ActiveRecord::Base.application_record(MyNamespace::MyModel) 
+    #       # => MyNamespace::ApplicationRecord
+    #
+    # If the class that is passed into <tt>application_record</tt> does not have
+    # a configuration associated with it, then the method will return the value
+    # that would have been returned without any argument. 
+    #
+    #   ActiveRecord::Base.application_reocrd('something_with_no_config')
+    #       # => ApplicationRecord
+    #
     module ClassMethods
       def configs_from(mod)
         app_record = self