db={"child":[{"_id":1,"group":"507f191e810c19729de860e1"},{"_id":2,"group":"507f191e810c19729de860e2"},{"_id":3,"group":"507f191e810c19729de860e3"},{"_id":4,"group":"507f191e810c19729de860e4"}],"parent":[{"_id":ObjectId("507f191e810c19729de860e1"),},{"_id":ObjectId("507f191e810c19729de860e2"),},{"_id":ObjectId("507f191e810c19729de860e3"),}]}

db.child.aggregate([{"$lookup":{"from":"parent","let":{"parent_group":{"$toObjectId":"$group"}},"pipeline":[{"$match":{"$expr":{"$eq":["$_id","$$parent_group"]}}}],"as":"parent_docs"}},{"$match":{"parent_docs":{"$size":0}}}])