[{"key":"good",bvn:11},{"key":"notemptystringisgood",bvn:"stringisnotempty"},{"key":"nullisbad",bvn:null},{"key":"emptystringisbad",bvn:""},{"key":"12toohigh",bvn:12},{"key":"10toolow",bvn:10},]

db.collection.find({$or:[{bvn:{$exists:false}},{bvn:""},{bvn:{$gt:11}},{bvn:{$lt:11}},]})