has_many :containing_relations, :class_name => "Relation", :through => :containing_relation_members, :source => :relation
validates :id, :uniqueness => true, :presence => { :on => :update },
has_many :containing_relations, :class_name => "Relation", :through => :containing_relation_members, :source => :relation
validates :id, :uniqueness => true, :presence => { :on => :update },