Database
[{"_id":"1","extra_fields":[{"tag":"alias","value":"thealias"},{"tag":"name","value":"nobody"},]},{"_id":"2","extra_fields":[{"tag":"alias","value":"thealias"},{"tag":"name","value":"nobody"},]},{"_id":"3","extra_fields":[{"tag":"noalias","value":"thealias"},{"tag":"name","value":"nobody"},]}]
Query
db.collection.find({"extra_fields.tag":"alias"})