Opensearch and Vega retrieve null fields in composite aggregation

Question

I'm using the VEGA visualization in OpenSearch to create a tree representation like this example from the documentation. Data are stored inside an index and I'm currently storing 3 fields related to this tree creation:

• Key (string)
• ParentKey (unique string of the parent) => equals to '-' for the root.
• Depth (int that represent to depth in the tree from 1 to x)

Key and depth are a primary key.

Here is how I'm formatting my data

  "data": [
    {
      name: "pageviews",
      "url":{
        %context%: true,
        index: "x"
        body:{
          "aggs": {
            "treeNodes": {
              "composite": {
                "size": 5000
                "sources": [
                  {
                    "key": {
                      "terms": {
                        "field": "x.key"
                      }
                    }
                  },
                  {
                    "lvl": {
                      "terms": {
                        "field": "x.lvl"
                      }
                    }
                  },
                 {
                    "parent": {
                      "terms": {
                        "field": "x.parentKey"
                      }
                    }
                  },
                ]
              }
            }
          }
        }
      }
    "format": {property: "aggregations.treeNodes.buckets"}
    "transform": [
        {
          "type": "stratify",
          "key": "key.key",
          "parentKey": "key.parent"
        },
        {
          "type": "tree",
          "method": "tidy",
          "size": ["1600", "500"],
          "separation": false,
          "as": ["y", "x", "depth", "children"]
        }
      ]
    },
    {
        "name": "links",
        "source": "x",
        "transform": [
          { "type": "treelinks" },
          {
            "type": "linkpath",
            "orient": "horizontal",
            "shape": "line",
          }
        ]
      }
  ],

As you can see, I'm using a composite aggregation that groups unique trio (key, parentKey, lvl).

But by doing so and with the tree example I'm getting the following error

missing: -

When I checked at the data from the example I could see that root node don't have any value for parent and so my problem is that I should not put any value for the parentKey of the root.

But I can't find a way to do it :

• If parentKey is null with my current aggregation, the value will be ignore and then I will miss the root node. So is there a way to add missing value ? I tried to do it by adding missing : null but missing seems to not be understand by composite aggregation.
• An other and probably better solution would be to exclude the parent in my aggregation (keep key + lvl) and then add the corresponding parentKey in a column. I'm new to all of this so I don't know how to do it.

Basically what I want to do is

Select parentKey, key, lvl from x group by key,lvl  

(It looks so basic in SQL..)

Hope someone can help I'm completely stuck.