How to project nested array elements with custom field name in MongoDB

Question

I have a users collection in MongoDB with this sample row:

[
    {
        "_id": 1,
        "name": "John Doe",
        "comments": [
            {
                "_id": 100,
                "content": "Comment 100",
                "post": "1000"
            },
            {   
                "_id": 101,
                "content": "Comment 101",
                "post": "1000"
            }
        ]
    }
]

I want to project users and comments data after converting _id fields into id. So I used the following query:

db.users.aggregate([
  {
    $project: {
      _id: 0,
      id: '$_id',
      name:1
      comments: {
        _id: 0,
        id: '$_id',
        content: 1,
      },
    }
  }
])

Now the users _id field is successfully converted. But the comments id field is equal to _id of users collection instead of comments.

[
  {
    "id": 1,
    "name": "john Doe",
    "comments": [
      {
        "id": 1,
        "content": "comment 100"
      },
      {
        "id": 1,
        "content": "comment 101"
      },
    ]
  }
]

How can I achieve the right result.

mohammad Naimi · Accepted Answer · 2021-11-02 07:49:40Z

1

you should use $map in project like this

https://mongoplayground.net/p/H4pueuejYdf

db.collection.aggregate([
  {
    "$project": {
      _id: 0,
      id: "$_id",
      name: 1,
      comments: {
        "$map": {
          "input": "$comments",
          "as": "c",
          "in": {
            id: "$$c._id",
            content: "$$c.content",
            post: "$$c.post"
          }
        }
      }
    }
  }
])

answered Nov 2, 2021 at 7:49

mohammad Naimi

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3 Comments

Saeid Asadi Over a year ago

Thanks. Doesn't this cause performance issues compared with simple $projection?

mohammad Naimi Over a year ago

So because your comments fields is array you couldnt change value as other value , there is another way to unwind project and then group , but need more process ,

mohammad Naimi Over a year ago

if my answer help you I appreciate to accept my answer

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