如何将$ project ObjectId转换为mongodb中的字符串值?

问题描述：

在 aggregate 函数中是否可以使用运算符来获取字符串而不是ObjectId?

Is there an operator I could use in aggregate function to get a string instead of ObjectId in response?

db.something.aggregate([
  { "$match": { "property": { "$exists": true } } },
  { "$project": { "stringId": "$_id.???" } }
])

答

Mongodb 4.0 已引入

Mongodb 4.0 has introduced $toString aggregation operator. So, Now you can easily convert ObjectId to string

db.collection.aggregate([
  {
    $project: {
      _id: {
        $toString: "$_id"
      }
    }
  }
])

反之亦然，使用 $toObjectId 聚合

db.collection.aggregate([
  {
    $project: {
      _id: {
        $toObjectId: "$_id"
      }
    }
  }
])