如何在Mongo中执行Stringified id和ObjectId之间的$lookup？

Question

情景

我必须在两个名为college和Student的集合上执行join。我使用的是MongoDB3.4。在这里，college有一个名为StudentId的字段，它引用学生集合的_id。问题是，studentID已经以字符串化形式存储，而_id是objectID。

db.college.aggregate(
    {
        $lookup: {
           from: "student",
           localField: "studentId",
           foreignField: "_id",
           as: "Related"
        }
    })

Answer 1

意图如下：

college(studentId)  --- > student (_id)
 string             ---->     ObjectId


we need to convert to ObjectId

    db.college.aggregate([
     {
 "$addFields": {
    "newStudentId" :  { "$toObjectId": "$studentId" }
       }
    },{
        $lookup: {
           from: "student",
           localField: "newStudentId",
           foreignField: "_id",
           as: "Related"
        }
    }])