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

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？

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"
  }}
])

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

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