$densify

Definition

Inserts documents for missing numeric or date positions.

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:

{
  "$densify": {
    "field": "x",
    "range": {
      "step": 1,
      "bounds": "full"
    }
  }
}

Parameters

Parameter	Required	Description
field	Yes	Numeric/date field path.
range.step	Yes	Positive increment.
range.bounds	No	"full" (default) or [lower, upper].
range.unit	For dates	"millisecond", "second", "minute", "hour", or "day".
partitionByFields	No	Nonempty array of field paths identifying independent partitions.

Behavior

• Processes gaps within each partition. Generated documents carry the densified field and partition fields.
• Null field values are rejected. Keep generation ranges bounded to avoid large allocations.

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 — $densify stage requires an object

Examples

Basic pipeline

Input documents:

[
  {
    "x": 1
  },
  {
    "x": 3
  }
]

Pipeline:

[
  {
    "$densify": {
      "field": "x",
      "range": {
        "step": 1,
        "bounds": "full"
      }
    }
  }
]

Result:

[
  {
    "x": 1
  },
  {
    "x": 2.0
  },
  {
    "x": 3
  }
]

Null field

Input documents:

[
  {
    "x": null
  }
]

Pipeline:

[
  {
    "$densify": {
      "field": "x",
      "range": {
        "step": 1
      }
    }
  }
]

Observed error: $densify field 'x' must have at least one non-null value

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.

• The "partition" bounds token and larger calendar units are not implemented. No live MongoDB stage parity is claimed.

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