+ belongs_to :old_relation, :foreign_key => [: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]
+ validates_length_of :v, :maximum => 255, :allow_blank => true