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$setField

The $setField operator allows adding, updating, or replacing a field in a document.

📌 Syntax

{
  "$setField": {
    "field": <fieldName>,
    "input": <document>,
    "value": <newValue>
  }
}
  • field: The field to set
  • input: The input document
  • value: The new value to assign

✅ Base Example – Set Field Value

📥 Input Document

{ "a": 1 }

📌 Expression

{
  "$setField": {
    "field": "b",
    "input": "$$ROOT",
    "value": 2
  }
}

📤 Output

{ "a": 1, "b": 2 }

✅ Overwrite Existing Field

📥 Input Document

{ "a": 10 }

📌 Expression

{
  "$setField": {
    "field": "a",
    "input": "$$ROOT",
    "value": 99
  }
}

📤 Output

{ "a": 99 }

🧱 Ecommerce Example – Add Rating to Product Attribute

📌 Pipeline

[
  {
    "$project": {
      "product": "$name",
      "updatedAttributes": {
        "$setField": {
          "field": "rating",
          "input": "$attributes",
          "value": 4.8
        }
      }
    }
  }
]

📥 Input Document

{
  "name": "Watch",
  "attributes": {
    "color": "Black",
    "style": "Sport"
  }
}

📤 Output

{
  "product": "Watch",
  "updatedAttributes": {
    "color": "Black",
    "style": "Sport",
    "rating": 4.8
  }
}

🔧 Common Use Cases

  • Modify nested subdocuments
  • Add calculated fields
  • Create documents with dynamic key-value pairs
  • $getField, $mergeObjects, $unsetField, $objectToArray

🧠 Notes

  • Allows modifying deeply nested fields with full control
  • Supports dynamic field names using expressions