参考集合的Mongo $ project _id

Question

我有以下文档结构：

{
 "_id": ObjectId("52caafa2eccc222244f242cc"),
 "fields": {
     $id: ObjectId("52caafa2eccc222244f242cc"),
     $db: 'other_db',
     $ref: CollectionName 
  }
}

我正在尝试将所有{id，fields.id}投影如下：

db.Collection.find({},{"_id": 1, "fields._id": 1}); 但没有任何结果

Answer 1

错误的文档结构，在阅读@yogesh建议的MongoDb Reference Documentation之后，问题是由$每个Reference键的SELECT title, COUNT(actorid) AS actors FROM movie JOIN casting ON id = movieid WHERE yr = 1978 GROUP BY title ORDER BY actors DESC 登录引起的。