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"