MONGOID-5780 was a response to a report that Foo.not.not.where was functionally equivalent to Foo.not.where – multiple nots did not negate each other, as would have been expected.

Unfortunately, the implementation of not (sans arguments) in Mongoid was not intended to be a toggle, and implementing it as a toggle broke some other expectations.

As a short-term fix, I think we should roll back that change. In the longer term, I've added MONGOID-5916 to address larger concerns around this parameterless not syntax.