$fill

Definition

Replaces null or missing values using a value expression or interpolation.

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:

{
  "$fill": {
    "sortBy": {
      "x": 1
    },
    "output": {
      "y": {
        "method": "linear"
      }
    }
  }
}

Parameters

Parameter	Required	Description
sortBy	Yes	Exactly one sort field with direction 1 or -1, including for value-based fill.
partitionBy	No	Expression grouping rows into independent partitions.
output	Yes	Nonempty object mapping destination fields to {value: expression} or {method: name}.
method	When interpolating	"locf" carries the last known value; "linear" interpolates between numeric endpoints.

Behavior

• Existing non-null values are preserved. Rows are sorted within each partition.
• Linear fill requires surrounding values; leading/trailing gaps without both endpoints remain unfilled.
• Use partitionBy; a partitionByFields option is not implemented in this handler.

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 — $fill stage requires an object

Examples

Basic pipeline

Input documents:

[
  {
    "x": 0,
    "y": 0
  },
  {
    "x": 1,
    "y": null
  },
  {
    "x": 2,
    "y": 10
  }
]

Pipeline:

[
  {
    "$fill": {
      "sortBy": {
        "x": 1
      },
      "output": {
        "y": {
          "method": "linear"
        }
      }
    }
  }
]

Result:

[
  {
    "x": 0,
    "y": 0
  },
  {
    "x": 1,
    "y": 5.0
  },
  {
    "x": 2,
    "y": 10
  }
]

Missing value fill

Input documents:

[
  {
    "x": 1
  }
]

Pipeline:

[
  {
    "$fill": {
      "sortBy": {
        "x": 1
      },
      "output": {
        "y": {
          "value": 0
        }
      }
    }
  }
]

Result:

[
  {
    "x": 1,
    "y": 0
  }
]

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 handler requires single-field sortBy and implements a subset of fill configuration.

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