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: Michael Koziarski <michael@koziarski.com>
Signed-off-by: José Valim <jose.valim@gmail.com>
Signed-off-by: Jeremy Kemper <jeremy@bitsweat.net>

Conflicts:

	activerecord/lib/active_record/associations/belongs_to_association.rb
	activerecord/lib/active_record/associations/has_one_association.rb