Can I set a field value as object key in MongoDB?

Question

I have a list of documents that looks like this:

_id: ObjectId('63c6a4b858612f44f37d4771')
type: "TypeA"
priority: 1

_id: ObjectId('627279d3ba7aef5d6418c867')
type: "TypeB"
priority: 2

The type here is unique so I can't have two documents with the same type. Now I want to set the type as the object name, with _id and priority inside the object. Like this:

TypeA: {
    _id: ObjectId('63c6a4b858612f44f37d4771'),
    priority: 1
}

TypeB: {
    _id: ObjectId('627279d3ba7aef5d6418c867'),
    priority: 2
}

I've tried something like this:

{
    $project: {
        '$$this.type': {
            _id: '$_id',
            priority: '$priority'
        }
    }
}

I just need help with getting the value of type as the object key.

Answer 1

To create a dynamic field name, create the object as an array of k v pairs, and use $arrayToObject like:

db.collection.aggregate([
  {$project: {
      newDoc: [ {k:"$type", v: {_id:"$_id", priority: "$priority"}} ]
  }},
  {$replaceRoot: {newRoot: {$arrayToObject: "$newDoc"}}}
])

Playground