mongodb, Updating a field in an object in array field equal to a document field

Question

let's say I have docs such as

{
  "nickname": "my nickname",
  "comments": [
    {
      "id": 1
    },
    {
      "id": 1
    }
  ]
}

how do I update it to look like

{
  "nickname": "my nickname",
  "comments": [
    {
      "id": 1,
      "nickname": "my nickname"
    },
    {
      "id": 1,
      "nickname": "my nickname"
    }
  ]
}

This does not seem to be working

db.getCollection('users').update(
{
    "comments.nickname": null
},
     { "$set": { "comments.$.nickname": "$nickname" } });

This is just an example to represent my problem.

I would not like to hear about re-structuring and optimizing the fields.

Thanks!

Valijon · Accepted Answer · 2020-02-02 02:54:31Z

2

Try this (v4.2):

db.users.updateMany(
    {"comments.nickname":null},
    [
        {"$set": {"comments.nickname": "$nickname"}}
    ]
)

Note: It will override if any comments.nickname already exists

db.users.aggregate([
  {
    $match: {
      "comments.nickname": null
    }
  },
  {
    $addFields: {
      comments: {
        $map: {
          input: "$comments",
          in: {
            id: "$$this.id",
            nickname: {
              $cond: [
                "$$this.nickname",
                "$$this.nickname",
                "$nickname"
              ]
            }
          }
        }
      }
    }
  },
  {
    $out: "users"
  }
])

Note: It will keep already existing values

edited Feb 2, 2020 at 2:54

answered Feb 2, 2020 at 2:23

Valijon

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