[
  {
    "id": "$group example",
    "stage": "$group",
    "kind": "example",
    "pipeline": [
      {
        "$group": {
          "_id": "$category",
          "total": {
            "$sum": "$amount"
          }
        }
      }
    ],
    "input": [
      {
        "category": "A",
        "amount": 2
      },
      {
        "category": "A",
        "amount": 3
      },
      {
        "category": "B",
        "amount": 1
      }
    ],
    "result": [
      {
        "_id": "A",
        "total": 5
      },
      {
        "_id": "B",
        "total": 1
      }
    ]
  },
  {
    "id": "$group empty input",
    "stage": "$group",
    "kind": "empty input",
    "pipeline": [
      {
        "$group": {
          "_id": "$category",
          "total": {
            "$sum": "$amount"
          }
        }
      }
    ],
    "input": [],
    "result": []
  },
  {
    "id": "$group invalid specification",
    "stage": "$group",
    "kind": "invalid specification",
    "pipeline": [
      {
        "$group": null
      }
    ],
    "input": [
      {
        "category": "A",
        "amount": 2
      },
      {
        "category": "A",
        "amount": 3
      },
      {
        "category": "B",
        "amount": 1
      }
    ],
    "error": "IllegalArgumentException",
    "message": "$group stage must be a document"
  },
  {
    "id": "$group null and missing group",
    "stage": "$group",
    "kind": "null and missing group",
    "pipeline": [
      {
        "$group": {
          "_id": "$category",
          "count": {
            "$sum": 1
          }
        }
      }
    ],
    "input": [
      {},
      {
        "category": null
      }
    ],
    "result": [
      {
        "_id": null,
        "count": 2
      }
    ]
  }
]