+ belongs_to :old_way, :query_constraints => [:way_id, :version], :inverse_of => :old_tags
+
+ validates :old_way, :associated => true
+ validates :k, :v, :allow_blank => true, :length => { :maximum => 255 }, :characters => true
+ validates :k, :uniqueness => { :scope => [:way_id, :version] }