db={"queries":[{"_id":1,"query":{"type":1,"subtype":1}},{"_id":2,"query":{"type":2,"subtype":1}}],"entries":[{"_id":1,"type":"1","subtype":"1","title":"1.1"},{"_id":2,"type":2,"subtype":1,"title":"2.1"},{"_id":3,"type":2,"subtype":2,"title":"2.2"}]}

db.queries.aggregate([{"$lookup":{"from":"entries","as":"entries","let":{"query":"$query"},"pipeline":[{"$match":"$$query"}]}}])