Providing support for :inverse_of as an option to associations.

You can now add an :inverse_of option to has_one, has_many and belongs_to associations.  This is best described with an example:

class Man < ActiveRecord::Base
  has_one :face, :inverse_of => :man
end

class Face < ActiveRecord::Base
  belongs_to :man, :inverse_of => :face
end

m = Man.first
f = m.face

Without :inverse_of m and f.man would be different instances of the same object (f.man being pulled from the database again).  With these new :inverse_of options m and f.man are the same in memory instance.

Currently :inverse_of supports has_one and has_many (but not the :through variants) associations.  It also supplies inverse support for belongs_to associations where the inverse is a has_one and it's not a polymorphic.

Signed-off-by: Murray Steele <muz@h-lame.com>
Signed-off-by: Jeremy Kemper <jeremy@bitsweat.net>

1 files changed, 21 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index 2d4c1d5507..ec0175497d 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -212,6 +212,13 @@ module ActiveRecord
       end
 
       def check_validity!
+        check_validity_of_inverse!
+      end
+
+      def check_validity_of_inverse!
+        if has_inverse? && inverse_of.nil?
+          raise InverseOfAssociationNotFoundError.new(self)
+        end
       end
 
       def through_reflection
@@ -225,6 +232,18 @@ module ActiveRecord
         nil
       end
 
+      def has_inverse?
+        !@options[:inverse_of].nil?
+      end
+
+      def inverse_of
+        if has_inverse?
+          @inverse_of ||= klass.reflect_on_association(options[:inverse_of])
+        else
+          nil
+        end
+      end
+
       private
         def derive_class_name
           class_name = name.to_s.camelize
@@ -300,6 +319,8 @@ module ActiveRecord
         unless [:belongs_to, :has_many].include?(source_reflection.macro) && source_reflection.options[:through].nil?
           raise HasManyThroughSourceAssociationMacroError.new(self)
         end
+
+        check_validity_of_inverse!
       end
 
       def through_reflection_primary_key