Mongoose aggregation lookup pipeline doesn't work

Question

Have any one idea why this aggregation only matches post, but doesn't populating comments?

I need user comments from comments collection, but only empty pipeline returns comments

Post.aggregate(...)

[
        {
            "$match": {
                "author": ObjectId(...)
            }
        },
        {
            "$lookup": {
                "from": "comments",
                "let": {
                    "postID": "$post",
                    "isHiden": "$isHiden"
                },
                "pipeline": [
                    {
                        "$match": {
                            "$expr": {
                                "$and": [
                                    {
                                        "$eq": [
                                            "$_id",
                                            "$$postID"
                                        ]
                                    },
                                    {
                                        "$eq": [
                                            "$$isHiden",
                                            0
                                        ]
                                    }
                                ]
                            }
                        }
                    }
                ],
                "as": "comments"
            }
        }
    ]

Comment object contains

{
  "_id": "5f7de8491af5c0e246d42609",
  "isHiden": false,
  "text": "...",
  "post": "5f7de8491af5c0e246d42605"
}

Post model is

{
  "_id": "5f7de8491af5c0e246d42605",
  "title": "Corporate Web Coordinator",
  "body": "...",
  "author": "5f7de8491af5c0e246d42602"
  }
}

I want to get result like:

       {
            "_id": "5f7de8491af5c0e246d42605",
            "confirm_status": "pending",
            "title": "Dynamic Marketing Supervisor",
            "body": "...",
            "author": "5f7de8491af5c0e246d42604",
            "comments": [
                  { "_id": "5f7de8491af5c0e246d42609",
                    "isHiden": false,
                    "text": "...",
                    "post": "5f7de8491af5c0e246d42605" }
             ]
        }

I tried everything, but nothing works ...

I'll appreciate any help

Answer 1

The problem is in your query,

• post field is present in comment table and you used _id, so it should be $post,
• inside pipeline field isHiden is not present in post table and you are defining that field in let, it should be,

    $lookup: {
      from: "comments",
      let: { postID: "$_id" },
      pipeline: [
        {
          $match: {
            $expr: [
              {
                $eq: [
                  "$$postID",
                  "$post"
                ]
              }
            ],
            isHiden: false
          }
        }
      ],
      as: "comments"
    }

Playground