mongodb（聚合）-带有查找结果的$lookup

Question

我是聚合框架的新手，有以下问题。

我的收藏是这样的：

用户

    _id: ObjectId('604caef28aa89e769585f0c4')
    baseData: { 
       firstName: "max"
       username: "max_muster"
       ...
       ...
    }

ActivitiesFeed

    _id: ObjectId('604cb97c99bbd77b54047370')
    userID: "604caef28aa89e769585f0c4"     // the user id 
    activity: "604caf718aa89e769585f0c8"   // the activity id 
    viewed: false 
    

活动

  _id: ObjectId('604caf718aa89e769585f0c8')
 "baseData": {
      "userID":"604caef28aa89e769585f0c4",
      "type":0,
      "title":"Alessandro send you a friend request.",
      "description":"",
      "actionID":"604caef28aa89e769585f0c4"
   },

聚合函数

const response = ActivityFeed.aggregate([
      {
        $match: { userID: ObjectId(args.user) },
      },
      {
        $lookup: {
          from: 'activities',
          localField: 'activity',
          foreignField: '_id',
          as: 'lookup_activities',
        },
      },
      { $unwind: '$activities' },
      {
        $lookup: {
          from: 'users',
          localField: 'lookup_activities.baseData.actionID',
          foreignField: '_id',
          as: 'lookup_users',
        },
      },
    ]);

如何使用第一个查找结果进行查找？

我查阅了activities集合。此集合保存第二个查找BaseData.UserID的id。但是使用这种方法，第二次查找没有结果。

我对activities集合进行连接的原因是actionID可以保存来自另一个集合的用户id或文档id。它取决于activities集合中的类型。

在聚合框架中是否可以进行依赖于前一个查找的类型和查找的查找？

提前谢谢你。

Answer 1

我所做的就是修复你的聚合体。您解出了错误的字段，请确保您的所有ID字段都是Objectid。如果您刚接触聚合功能，我强烈建议您使用MongoDB Compass来帮助您的聚合。他们有一个阶段一个阶段的辅助图形界面，这将真正使您的生活更容易。

蒙古游乐场

db.activitiesFeed.aggregate([
  {
    $match: {
      userID: ObjectId("604caef28aa89e769585f0c4")
    },
  },
  {
    $lookup: {
      from: "activities",
      localField: "activity",
      foreignField: "_id",
      as: "lookup_activities",
    },
  },
  {
    $unwind: "$lookup_activities"
  },
  {
    $lookup: {
      from: "users",
      localField: "lookup_activities.baseData.actionID",
      foreignField: "_id",
      as: "lookup_users",
    },
  },
])

使用的示例

 db={
  "users": [
    {
      _id: ObjectId("604caef28aa89e769585f0c4"),
      baseData: {
        firstName: "max",
        username: "max_muster"
      }
    }
  ],
  "activitiesFeed": [
    {
      _id: ObjectId("604cb97c99bbd77b54047370"),
      userID: ObjectId("604caef28aa89e769585f0c4"),
      activity: ObjectId("604caf718aa89e769585f0c8"),
      viewed: false
    }
  ],
  "activities": [
    {
      _id: ObjectId("604caf718aa89e769585f0c8"),
      "baseData": {
        "userID": ObjectId("604caef28aa89e769585f0c4"),
        "type": 0,
        "title": "Alessandro send you a friend request.",
        "description": "",
        "actionID": ObjectId("604caef28aa89e769585f0c4")
      },
    }
  ]
}