validates_presence_of :old_relation
validates_length_of :k, :maximum => 255, :allow_blank => true
validates_uniqueness_of :k, :scope => [:relation_id, :version]
validates_presence_of :old_relation
validates_length_of :k, :maximum => 255, :allow_blank => true
validates_uniqueness_of :k, :scope => [:relation_id, :version]