[
  {
    "id": "$sortByCount example",
    "stage": "$sortByCount",
    "kind": "example",
    "pipeline": [
      {
        "$sortByCount": "$category"
      }
    ],
    "input": [
      {
        "category": "A"
      },
      {
        "category": "A"
      },
      {
        "category": "B"
      }
    ],
    "result": [
      {
        "_id": "A",
        "count": 2
      },
      {
        "_id": "B",
        "count": 1
      }
    ]
  },
  {
    "id": "$sortByCount empty input",
    "stage": "$sortByCount",
    "kind": "empty input",
    "pipeline": [
      {
        "$sortByCount": "$category"
      }
    ],
    "input": [],
    "result": []
  },
  {
    "id": "$sortByCount invalid specification",
    "stage": "$sortByCount",
    "kind": "invalid specification",
    "pipeline": [
      {
        "$sortByCount": null
      }
    ],
    "input": [
      {
        "category": "A"
      },
      {
        "category": "A"
      },
      {
        "category": "B"
      }
    ],
    "error": "IllegalArgumentException",
    "message": "$sortByCount requires a grouping expression"
  }
]