https://github.com/mongoid/mongoid/blob/8b33b62a43fb989a6a6db9beb72d98d8c84874ea/lib/mongoid/validatable.rb#L79

The method #read_attibute_for_validation is supposed to return the value used for validation. Thus it should return the raw value and not utilize the getter from send(attr).

It crops up as a problem in Rails https://github.com/rails/rails/blob/a3b1105ada3da64acfa3843b164b14b734456a50/activemodel/lib/active_model/validator.rb#L165 whenever you use allow_nil or allow_blank on the field and are overriding the getter.

Since the raw value is what should be used for validation, I would argue that instead of send(attr) it should return attributes[attribute].