mongodb – 如何在$lookup(聚合)中将ObjectID转换为String

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,那怎么办？

您可以使用 ​​$toObjectId​​聚合实现此目的,该聚合仅将字符串ID转换为mongoose 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"
  }}
])

或者使用​​$toString​​聚合

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