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？
$toObjectId聚合实现此目的,该聚合仅将字符串ID转换为mongoose objectId
db.collection(‘article‘).aggregate([ { "$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" }} ])
mongodb – 如何在$lookup(聚合)中将ObjectID转换为String
原文：https://www.cnblogs.com/cangqinglang/p/14858131.html