I want to split the following array according to the group-value. I know I can do this using `$unwind` and `$group`. Is there any way to this in a single `$project`-stage?

Input

```
{
    "_id": 1,
    "some_field": "some_value",
    "array": [
        {
            "group": "a",
            "subgroup": "aa",
            "value": 1
        },
        {
            "group": "b",
            "subgroup": "bb",
            "value": 2
        },
        {
            "group": "a",
            "subgroup": "ab",
            "value": 2
        }
    ]
}

```

desired output:

```
{
    "_id": 1,
    "some_field": "some_value",
    "array": [
        {
            "group": "a",
            "values": [
                {
                    "subgroup": "aa",
                    "value": 1
                },
                {
                    "subgroup": "ab",
                    "value": 2
                }
            ]
        },
        {
            "group": "b",
            "values": [
                {
                    "subgroup": "bb",
                    "value": 2
                }
            ]
        }
    ]
}

```