$setWindowFields

Definition

Adds values computed over an ordered partition of documents.

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

Syntax

Basic form, with concrete values:

{
  "$setWindowFields": {
    "sortBy": {
      "score": 1
    },
    "output": {
      "rank": {
        "$rank": {}
      }
    }
  }
}

Parameters

Parameter	Required	Description
partitionBy	No	Expression identifying independent partitions.
sortBy	Yes	Exactly one field with numeric direction 1 or -1.
output	Yes	Nonempty object of output fields, each containing one supported window function.
window.documents	No	Two bounds: integer offsets, "current", or "unbounded". Default is unbounded-to-current.

Behavior

• Supported functions: $shift, $rank, $denseRank, $derivative, $integral, $expMovingAvg, $stdDevPop, $stdDevSamp, $covariancePop, $covarianceSamp.
• Only document-position windows are implemented. Range/time windows are rejected.
• A function supported as an expression or grouping accumulator is not automatically a supported window function.

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 — $setWindowFields stage expects an object

Examples

Basic pipeline

Input documents:

[
  {
    "score": 20
  },
  {
    "score": 10
  },
  {
    "score": 20
  }
]

Pipeline:

[
  {
    "$setWindowFields": {
      "sortBy": {
        "score": 1
      },
      "output": {
        "rank": {
          "$rank": {}
        }
      }
    }
  }
]

Result:

[
  {
    "score": 20,
    "rank": 2
  },
  {
    "score": 10,
    "rank": 1
  },
  {
    "score": 20,
    "rank": 2
  }
]

Unsupported sum

Input documents:

[
  {
    "score": 10
  }
]

Pipeline:

[
  {
    "$setWindowFields": {
      "sortBy": {
        "score": 1
      },
      "output": {
        "total": {
          "$sum": "$score"
        }
      }
    }
  }
]

Observed error: Unsupported window function: $sum

Verification

The 4 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.

• This is a subset of MongoDB window functionality; range windows and other window accumulators are not implemented.

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