$lookup 无法在 mongodb 中显示非相关记录

Question

我正在尝试将 $lookup 用于两个表，但在查找后我也得到了不相关的记录。

表用户

{"_id" : ObjectId("5c1f5fe31e11011268006aa7"),"name" : "v","email" : "v@gmail.com"},
{"_id" : ObjectId("5c1f5fd31e11011268006aa6"),"name" : "r","email" : "r@gmail.com",}

订桌

schema like 
(student_id: { type: Schema.Types.ObjectId, ref: 'users' },
teacher_id: { type: Schema.Types.ObjectId, ref: 'users' })

{
    "_id" : ObjectId("5c1f642aa6fb4511a0511e41"),
    "student_id" : ObjectId("5c1f5fd31e11011268006aa7"),
    "createdAt" : ISODate("2018-12-23T16:02:10.179+05:30"),
    "updatedAt" : ISODate("2018-12-23T16:02:10.179+05:30"),
    "__v" : 0,
}

$lookup

db.bookings.aggregate(
   {
     $lookup:
       {
         from: "users",
         localField: "ObjectId(student_id)",
         foreignField: "ObjectId(_id)",
         as: "student_info"
       }
  }
)

结果我得到了两个用户

{
    "_id": "5c1f642aa6fb4511a0511e41",
    "student_id": "5c1f5fd31e11011268006aa7",
    "teacher_id": "5c1f5fe31e11011268006aa6",
    "createdAt": "2018-12-23T10:32:10.179Z",
    "updatedAt": "2018-12-23T10:32:10.179Z",
    "__v": 0,
    "student": [
        {
            "_id": "5c1f5fd31e11011268006aa6",
            "name": "r",
            "email": "a@gmail.com"
        },
        {
            "_id": "5c1f5fe31e11011268006aa7",
            "name": "v",
            "email": "v@gmail.com"
        }
    ]
}

但是如果我使用 localField: "student_id", ForeignField: "_id"，在查找中我得到的是空白的 student_info 数组

Answer 1

您的localField 和foreignField 是文档本身的字段。 您使用的 ("ObjectId(student_id)") 只是一个字符串，并且在文档中不存在任何具有该名称的字段。

db.bookings.aggregate(
   {
     $lookup:
       {
         from: "users",
         localField: "student_id",
         foreignField: "_id",
         as: "student_info"
       }
  }
)