Question

我有两个集合，文章和评论，评论中的文章是文章中_id的外键。

        db.collection('article').aggregate(
          [
            {
              $lookup:
                {
                  from: "comments",
                  localField: "_id",
                  foreignField: "articleId",
                  as: "comments"
                }
            },
          ...

它不起作用，因为文章中的_id是一个ObjectID而articleId是字符串，那该怎么办？

Answer 1

您可以使用$toObjectId聚合来实现此目的，该聚合只需将字符串id转换为猫鼬objectId

db.collection('article').aggregate([
  { "$lookup": {
    "from": "comments",
    "let": { "article_Id": "$_id" },
    "pipeline": [
      { "addFields": { "articleId": { "$toObjectId": "$articleId" }}},
      { "$match": { "$expr": { "$eq": [ "$articleId", "$$article_Id" ] } } }
    ],
    "as": "comments"
  }}
])

或使用$toString聚合

db.collection('article').aggregate([
  { "addFields": { "article_id": { "$toString": "$_id" }}},
  { "$lookup": {
    "from": "comments",
    "localField": "article_id",
    "foreignField": "articleId",
    "as": "comments"
  }}
])

Answer 2

您可以在$toString字段上直接使用_id：

db.collection('article').aggregate([
  {
    $lookup: {
      from: "comments",
      localField: { $toString : "_id" },
      foreignField: "articleId",
      as: "comments"
    }
  },
  ...
])

但是MongoDB 3.6不支持聚合管道中的类型转换。因此$toString和$convert仅适用于MongoDB 4.0。

如何在$ lookup（聚合）中将ObjectID转换为String

2 个答案: