[SOLVED] Using JSONata to group by a property value

Using JSONata to group by a property value

I would like to use JSONata to reformat a JSON response into a different structure grouped by values in the object. I've searched StackOverflow but cannot see an example that matches my needs. I would like to turn this:

[
    {
        "group": "a",
        "month": "01",
        "value": "10"
    },
    {
        "group": "a",
        "month": "02",
        "value": "20"
    },
    {
        "group": "b",
        "month": "01",
        "value": "30"
    },
    {
        "group": "b",
        "month": "02",
        "value": "40"
    }
]

into this

{
   "a": {
      "01": "10",
      "02": "20"
   },
   "b": {
      "01": "30",
      "02", "40"
   }
}

The group values and months will change over time so it needs to be dynamic. Can anyone help?

Update: This is what I've tried so far which gets me close but not the exact structure I'm looking for:

${
    $string(group): $.({
        month: $.value
    })
}

The result groups correctly but the months are in an array instead of an object:

{
  "a": [
    {
      "01": "10"
    },
    {
      "02": "20"
    }
  ],
  "b": [
    {
      "01": "30"
    },
    {
      "02": "40"
    }
  ]
}

Solution

How about:

${group: {month: value}}

This groups the initial array by the group value. Then, the resulting array for each group is in turn grouped by month.

Interestingly, JSONata understands that there is only one item per group now, and implicitly chooses the correct aggregation function, which simply returns the item.