I am still on mongoid 2.x so have only actually tested with that, although I did just take a peek at master and I think the problem is still present there. But apologies if not.

If you look at Criteria#method_missing, if the method call gets delegated to entries, any block is not passed on. This means things like:

Foo.all.uniq(&:field_to_be_uniqed)

don't work. Obviously you can just bung a to_a or whatever in there, but obviously one would expect it work without.

Cheers.