MongoDB: Aggregation to convert ObjectId to DBRef

Isit possible to have an aggregation that converts an objectId column into a dbRef? I am attempting to use aggregation and $merge to transform 1 table with an ObjectId column into a DBRef one

{
    $addFields: {
        name: "$field",
        parentId: {
            $cond: {
                if: { $ne: ["$parentId", null] },
                then: DBRef("parents", "$parentId"),
                else: null
            }
        }
    },
},

tried this but got

> An object representing an expression must have exactly one field

I think it is. Internally DBRefs are stored in this structure in MongoDB,

{ "$ref" : <value>, "$id" : <value>, "$db" : <value> }

So when you execute your query, the parentId becomes of this form, and you get an error because MongoDB interprets any key beginning with $ to be an operator in the aggregation pipeline. You can overcome this using, $setField like this:

db.collection.aggregate([
  {
    "$addFields": {
      "parentIdCopy": {
        $cond: {
          if: {
            $ne: [
              "$parentId",
              null
            ]
          },
          then: {
            "$setField": {
              "field": {
                $literal: "$ref"
              },
              "input": {},
              "value": "parents"
            },
            
          },
          else: null
        }
      }
    }
  },
  {
    "$addFields": {
      "parentIdCopy": {
        $cond: {
          if: {
            $ne: [
              "$parentId",
              null
            ]
          },
          then: {
            "$setField": {
              "field": {
                $literal: "$id"
              },
              "input": "$parentIdCopy",
              "value": "$parentId"
            },
            
          },
          else: null
        }
      }
    }
  },
  {
    "$set": {
      "parentId": "$parentIdCopy",
      parentIdCopy: "$$REMOVE"
    }
  },
  {
    "$merge": {
      "into": "collection",
      "on": "_id",
      "whenMatched": "replace",
      
    }
  }
])

In this query, we are manually generating the object in DBRef format, rather than using the constructor. Playground link.