$set

Definition

Adds or replaces fields while retaining other fields.

Core aggregation stage · Reshape documents. Use this syntax in a Core pipeline; Studio availability depends on the Flow compiler.

Syntax

Basic form, with concrete values:

{
  "$set": {
    "total": {
      "$multiply": [
        "$quantity",
        "$price"
      ]
    }
  }
}

Parameters

Parameter	Required	Description
field: expression	Yes	Destination field path and expression. A dotted path writes a nested object field.

Behavior

• One output document is returned for each input document.
• Direct field expressions read the incoming document. Use a later stage when one assignment depends on a previous assignment.
• Nested writes use shallow document copies in this handler. Do not rely on direct Core stage calls to provide SrotaX’s immutable-input authoring guarantees.

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 — $set expects an object

Examples

Basic pipeline

Input documents:

[
  {
    "quantity": 2,
    "price": 5,
    "orderId": "A"
  }
]

Pipeline:

[
  {
    "$set": {
      "total": {
        "$multiply": [
          "$quantity",
          "$price"
        ]
      }
    }
  }
]

Result:

[
  {
    "quantity": 2,
    "price": 5,
    "orderId": "A",
    "total": 10.0
  }
]

Null and missing source

Input documents:

[
  {},
  {
    "value": null
  }
]

Pipeline:

[
  {
    "$set": {
      "copy": "$value"
    }
  }
]

Result:

[
  {
    "copy": null
  },
  {
    "value": null,
    "copy": null
  }
]

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.

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