[{"collection":"collection","count":3,"content":{"cheese":{"type":"enum","values":[null]},"butter":{"type":"enum","values":[1,2,3]}},"indexes":[{"name":"cheese_idx","key":{"cheese":1,"butter":1}}]}]

db.collection.aggregate([{$match:{cheese:null}},{$project:{_id:0,butter:1}}]).explain("queryPlanner")