Mongodb 本地字段和外来字段不工作的查找管道内的项目

我已经写了下面几行代码来获取查找中的一些特定字段，如

       $pipeline = array(
            array(
                '$match' => $query
            ),
            array(
                '$lookup' => array(
                    'from' => 'studentTbl',
                    'localField' => '_id',
                    'foreignField' => 'activity_details.activityId',
                     'pipeline' => [
                        ['$project' => [ '_id' =>  1.0, 'activity_details' => 1.0] ],
                     ],   
                    'as' => 'studentsOfActivities'
                )
            ),
           ....
           ....
        );

     return $this->db->activitiesTbl->aggregate($pipeline)->toArray();

基本上studentTbl有许多字段和嵌入式文档。在上面的代码中，我首先使用foreign和local字段进行查找，然后确定哪些字段应该投影到管道中

---

上面的代码不起作用。。。请帮忙

您可以使用下面的聚合

db.collection.aggregate([
  { "$match": $query },
  { "$lookup": {
    "from": "studentTbl",
    "let": { "activityId": "$_id" },
    "pipeline": [
      { "$match": { "$expr": { "$in": ["$$activityId", "$activity_details.activityId"] }}},
      { "$project": { "activity_details": 1 }}
    ],
    "as": "studentsOfActivities"
  }}
])

---

你好，如果你们看起来像这样，这里是答案： 在MongoDB的侧面聚合方法中使用带有本地字段和外来字段的项目

db.collection.aggregate([
                {
                    $lookup: {
                        from: "users", // collection name in db
                        localField: "user_id",
                        foreignField: "_id",                           
                        as: "userdata",                            
                    },

                },
                {$match :{_id: {$in:losers} }},
                {$project:
                    {
                      "answer":1,"_id":1,"win_status":1,"user_id":1,"user_id":1,
                        "userdata":{"name":1,"image":1,"email":1},
                    }
                }
            ])

---

activity\u details

是数组还是对象？它在数组中（studentTbl的嵌入式文档）