Trouble transforming _source field in VEGA for hits.hits formatted data

Elastic Stack Kibana
1

This seems to be a common problem. Is there documentation or an example someone could point me to?

My VEGA schema contains a format for non-aggregate query results

  data: {
    url: {
      index: myIndex
      "body": {
        "size": 10, "_source": ["objectField", "dateField"]
      }
    },
    "format": {"property": "hits.hits"}
  }

So the VEGA Debug portion shows the _source field in the table as expected.

  "hits": {
    "total": 7,
    "max_score": 1,
    "hits": [
      {
        "_index": "myIndex",
        "_id": "20250824",
        "_score": 1,
        "_source": {
          "dateField": "2025-08-24T00:00:00Z",
          "objectField": [
            {
              "count": 9298,
              "id": "202509011839-0"
            },
            {
              "count": 8512,
              "id": "202509011038-0"
            },

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image1722×666 75 KB

Naturally, we would like to unpack this data using a transform. The _source field is an array of objects, and datum allows us to access the VEGA debug’s table column names. So let’s try using the project transformation.

  transform: [
    {
      "type": "project",
      "fields": [
        "datum._source.dateField",
        "datum._source.objectField",
      ],
      "as": ["dateField", "objectField"]
    }
  ]

My problem is I expect to see the new object fields in my VEGA debug, but nothing changed.

I’ve also tried calculate as suggested in Vega Boxplot - referencing properties in _source (infinite extent error)

    {
      calculate: "datum._source.dateField",
      as: "dateField"
    }

But still, nothing happens.