聚合和 $lookup

Question

我在 MongoDB 中有两个集合，我想从中生成一个报告。采集数据如下 学生

[
    {
        _id:"1",
        name:"stu 1",
        class: "7th"
    },
    {
        _id:"2",
        name:"stu 2",
        class: "7th"
    },
    {
        _id:"3",
        name:"stu 1",
        class: "7th"
    }
]

活动

[
    {
        _id:"1",
        studentIds:["1","3"],
        taskName:"Craft work 1"
    },
    {
        _id:"2",
        studentIds:["1"],
        taskName:"Craft work 2"
    }

]

我想使用聚合管道生成的报告如下所示，我对 $loockup 管道感到震惊。有人可以帮我解决这个问题吗？

学生报告

{
    _id:1
    name: "stu 1",
    class: "7th",
    tasks: ["Craft work 1", "Craft work 2"] 
},
{
    _id:1
    name: "stu 2",
    class: "7th",
    tasks: []   
},
{
    _id:1
    name: "stu 3",
    class: "7th",
    tasks: ["Craft work 1", "Craft work 2"] 
}

Answer 1

我假设活动中的 studentIds 字段应该是一个数组。这似乎是一个错字。如果是这样，您可以使用以下方式聚合所需的输出：

https://mongoplayground.net/p/UBE0oNWLBsR

db.students.aggregate([
  {
    $lookup: {
      "from": "activities",
      "localField": "_id",
      "foreignField": "studentIds",
      "as": "tasks"
    }
  },
  {
    $addFields: {
      tasks: "$tasks.taskName"
    }
  }
])