class User include MongoMapper::Document end class Address include MongoMapper::EmbeddedDocument embedded_in: user key :city validates_presence_of :user end user.address.city = nil user.address.valid? #=> false user.valid? # => true user.save! #=> does not raise error user.reload user.address.city #=> nil 本家の issue No validation in EmbbededDocument · Issue #109 · mongomapper/mongomapper · GitHub で