Database
[{"a":"Bahrain","b":"Bahrain","c":"Bahrain","d":"Oman","e":"Oman","f":"Oman"}]
Query
db.collection.aggregate([{$match:{}},{$project:{fieldKeys:{$map:{input:{$filter:{input:{"$objectToArray":"$$ROOT"},as:"datum",cond:{$eq:["$$datum.v","Bahrain"]}}},as:"key",in:"$$key.k"}}}}])