[{"_id":ObjectId("5c09f570173f7900015a82b2"),"conversations":{"0":{"message":"Message 4"}}},{"_id":ObjectId("5c09a454dd42360001fd2515"),"conversations":{"0":{"message":"Message 1"},"1":{"message":"Message 1"},"2":{"message":"Message 5"},"3":{"message":"Message 1"},"4":{"message":"Message 2"},"5":{"message":"Message 3"},"6":{"message":"compliance"},"7":{"message":"Google"}}}]

db.collection.aggregate([{$match:{$expr:{$ne:[{$size:{$filter:{input:{$objectToArray:"$conversations"},as:"conv",cond:{$eq:["$$conv.v.message","Message 4"]}}}},0]}}}])