$sortByCount

Definition

Counts documents per grouping value and sorts groups by descending count.

Core aggregation stage · Group and summarize. Use this syntax in a Core pipeline; Studio availability depends on the Flow compiler.

Syntax

Basic form, with concrete values:

{
  "$sortByCount": "$category"
}

Parameters

Parameter	Required	Description
expression	Yes	Expression used as the grouping key.

Behavior

• Internally executes $group with a count followed by $sort on count descending.
• Output fields are _id and count. Equal-count ordering is not a business ordering guarantee.

Empty input and invalid specification

These cases use the basic form above unless the specification is explicitly replaced.

Case	Observed result
empty input	[]
invalid specification (null)	Error: IllegalArgumentException — $sortByCount requires a grouping expression

Examples

Basic pipeline

Input documents:

[
  {
    "category": "A"
  },
  {
    "category": "A"
  },
  {
    "category": "B"
  }
]

Pipeline:

[
  {
    "$sortByCount": "$category"
  }
]

Result:

[
  {
    "_id": "A",
    "count": 2
  },
  {
    "_id": "B",
    "count": 1
  }
]

Verification

The 3 examples on this page were executed through Core PipelineExecutor with a fresh context for each case. They establish the displayed behavior, not all possible option combinations.

Download the executable cases and recorded results.

MongoDB differences

Stage examples were executed in Core. They were not compared live with MongoDB in the expression parity suite.

MongoDB $sortByCount reference. Read the Core behavior above before reusing MongoDB examples.