toObjectId，用于将字符串的字段与查找和聚合一起使用

问题描述

我有2个模型，一个是小组，另一个是学生.组看起来像这样

I have 2 models, one is Group and another is Students. Group looks like this

{
    "_id" : ObjectId("5c8d28ef7e0e542854b7b904"),
    "name" : "Homeroom ",
    "year" : 2019,
    "schoolID" : ObjectId("5c1a735fc98da061141475a1"),
    "teachers" : [ 
        {
            "_id" : "5c1a7677c98da061141475aa",
            "firstName" : "Ayam"
        }, 
        {
            "_id" : "5c1a7677c98da061141475a9",
            "firstName" : "Itik"
        }
    ],
    "addedOn" : ISODate("2019-03-16T16:48:47.372Z"),
    "lastModified" : ISODate("2019-03-16T16:48:47.372Z"),
    "__v" : 0,
    "status" : 1,
    "students" : [ 
        "5c1a79f7c98da061141475b7", 
        "5c3bfea37774fb0b55000cb5", 
        "5c1a7c69c98da061141475bb", 
        "5c3bfea37774fb0b55000cb4", 
        "5c1a7d32c98da061141475be", 
        "5c3bfea37774fb0b55000cb7"
    ]
}

上面的字段students存储学生模型中学生的_id(以String格式).

Where the field students above stores the _id (in String format) of students inside Students model.

现在我正在尝试使用聚合进行查找，并且我想到了这样的东西:

Now I'm trying to do a lookup with aggregate, and I comes up with something like this:

Group.aggregate([
            { $match: { _id: mongoose.mongo.ObjectId(groupID) } },
            { $lookup: { 
              from: "Students", localField: "students", foreignField: "_id", as: "studentList"  
            } },
              { $unwind: "$studentList" },
              { $replaceRoot: { newRoot: "$students" } }
          ], function(err, result){
            if (err){
              console.log("imin err 102: " )
              console.log(err)
            }else{
              console.log("imini 105 result")
              console.log(result);
            }
          });

现在，我知道上面的代码不会产生任何结果，因为模型组中的students存储为字符串，而模型Student中的_id是ObjectId. Mongodb现在有了$toObjectId，但是由于我的模型拥有一个String数组，所以我不知道如何正确实现$toObjectId.

Now I understand that the code above won't yield any result, since students inside Model Group is stored as String, while the _id inside model Student is an ObjectId. Mongodb now have $toObjectId but since my Model holds an array of String, I don't know how to implement $toObjectId correctly.

这里是学生的示例文件

{
    "_id" : ObjectId("5c1a79f7c98da061141475b7"),
    "firstName" : "Ibrahim",
    "kelasID" : ObjectId("5c429f9906f2a805bc6cd494"),
    "lastName" : "Ali",
    "schoolID" : ObjectId("5c1a735fc98da061141475a1"),
    "year" : 2018,
    "__v" : 0,
    "addedOn" : ISODate("2018-12-25T04:27:47.909Z"),
    "checkIn" : false,
    "checkInStatus" : 1,
    "contactNo1" : "012225656",
    "father" : "Ali",
    "fatherID" : "8852245",
    "idType" : 0,
    "lastModified" : ISODate("2018-12-25T04:27:47.909Z"),
    "mother" : "",
    "motherID" : ""
}
{
    "_id" : ObjectId("5c3bfea37774fb0b55000cb5"),
    "idType" : 0,
    "checkIn" : false,
    "checkInStatus" : 1,
    "year" : 2019,
    "schoolID" : ObjectId("5c1a735fc98da061141475a1"),
    "kelasID" : ObjectId("5c1a7534c98da061141475a3"),
    "firstName" : "Umar",
    "lastName" : "Bin Al-Khattab",
    "contactNo1" : "601222",
    "status" : 1,
    "addedOn" : ISODate("2019-01-14T03:14:43.597Z"),
    "lastModified" : ISODate("2019-01-14T03:14:43.597Z"),
    "__v" : 0
}
{
    "_id" : ObjectId("5c1a7c69c98da061141475bb"),
    "idType" : 0,
    "checkIn" : false,
    "checkInStatus" : 1,
    "year" : 2018,
    "schoolID" : ObjectId("5c1a735fc98da061141475a1"),
    "kelasID" : ObjectId("5c1a7540c98da061141475a5"),
    "firstName" : "Abdul Rahman",
    "lastName" : "Affan",
    "father" : "Affan",
    "fatherID" : "54321",
    "contactNo1" : "602288",
    "status" : 1,
    "addedOn" : ISODate("2018-12-25T04:30:16.130Z"),
    "lastModified" : ISODate("2018-12-25T04:30:16.130Z"),
    "__v" : 0
}

推荐答案

您必须 students字段上的"> $map ，将String id转换为ObjectId

You have to $map over the students field to convert the String ids to ObjectId

Group.aggregate(
  [
    { "$match": { "_id": mongoose.mongo.ObjectId(groupID) } },
    { "$addFields": {
      "students": {
        "$map": {
          "input": "$students",
          "in": { "$toObjectId": "$$this" }
        }
      }
    }},
    {
      "$lookup": {
        "from": "Students",
        "localField": "students",
        "foreignField": "_id",
        "as": "studentList"
      }
    },
    { "$unwind": "$studentList" },
    { "$replaceRoot": { "newRoot": "$students" } }
  ],
  function(err, result) {
    if (err) {
      console.log("imin err 102: ");
      console.log(err);
    } else {
      console.log("imini 105 result");
      console.log(result);
    }
  }
)

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