Mongodb加入从String到ObjectId的_id字段 [英] Mongodb Join on _id field from String to ObjectId

问题描述

我有两个收藏

1. 用户

{
   "_id" : ObjectId("584aac38686860d502929b8b"),
   "name" : "John"
}

角色

Role

{
   "_id" : ObjectId("584aaca6686860d502929b8d"),
   "role" : "Admin",
   "userId" : "584aac38686860d502929b8b"  
}

我想根据 userId(在 role 集合中)- _id(在 user 中加入这些集合> 收藏).

I want to join these collection based on the userId (in role collection) - _id ( in user collection).

我尝试了以下查询:

db.role.aggregate({
  "$lookup": {
    "from": "user",
    "localField": "userId",
    "foreignField": "_id",
    "as": "output"
  }
})

只要我将 userId 存储为 ObjectId，这就会给我预期的结果.当我的 userId 是字符串时，没有结果.ps:我试过了

This gives me expected results as long as i store userId as a ObjectId. When my userId is a string there are no results. Ps: I tried

foreignField: '_id'.valueOf()

foreignField: '_id'.valueOf()

和

foreignField: '_id'.toString()

foreignField: '_id'.toString()

.但是根据 ObjectId 字符串字段匹配/加入没有运气.

. But no luck to match/join based on a ObjectId-string fields.

任何帮助将不胜感激.

推荐答案

这在 MongoDB 3.4 中是不可能的.此功能已被请求，但尚未实施.以下是相应的门票:

This is not possible as of MongoDB 3.4. This feature has already been requested, but hasn't been implemented yet. Here are the corresponding tickets:

• SERVER-22781:允许在 ObjectId (_id.str) 和字符串
• SERVER-24947:需要布尔类型转换机制，ISODates，对象ID

现在您必须将 userId 存储为 ObjectId

For now you'll have to store userId as ObjectId

编辑

以前的票证在 MongoDB 4.0 中已修复.您现在可以使用以下查询实现此目的:

The previous tickets were fixed in MongoDB 4.0. You can now achieve this with the folowing query:

db.user.aggregate([
  {
    "$project": {
      "_id": {
        "$toString": "$_id"
      }
    }
  },
  {
    "$lookup": {
      "from": "role",
      "localField": "_id",
      "foreignField": "userId",
      "as": "role"
    }
  }
])

结果:

[
  {
    "_id": "584aac38686860d502929b8b",
    "role": [
      {
        "_id": ObjectId("584aaca6686860d502929b8d"),
        "role": "Admin",
        "userId": "584aac38686860d502929b8b"
      }
    ]
  }
]

在线试用:mongoplayground.net/p/JoLPVIb1OLS

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