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$getField

The $getField operator retrieves the value of a specified field from a document, supporting both static and dynamic keys.

📌 Syntax

{
  "$getField": {
    "field": <fieldName>,
    "input": <document>
  }
}
  • field: Name of the field to access (string or expression)
  • input: The document to read from (optional — defaults to current document)

✅ Base Example – Static Field Name

📥 Input Document

{ "a": 10, "b": 20 }

📌 Expression

{
  "$getField": {
    "field": "a"
  }
}

📤 Output

10

✅ Dynamic Field from Variable

📥 Input Document

{ "stats": { "score": 88 }, "target": "score" }

📌 Expression

{
  "$getField": {
    "field": "$target",
    "input": "$stats"
  }
}

📤 Output

88

🧱 Ecommerce Example – Access Variant Detail by Dynamic Key

📌 Pipeline

[
  {
    "$project": {
      "variantColor": {
        "$getField": {
          "field": "$selectedColor",
          "input": "$variants"
        }
      },
      "product": "$name"
    }
  }
]

📥 Input Document

{
  "name": "Backpack",
  "selectedColor": "blue",
  "variants": {
    "red": { "stock": 5 },
    "blue": { "stock": 12 }
  }
}

📤 Output

{
  "product": "Backpack",
  "variantColor": { "stock": 12 }
}

🔧 Common Use Cases

  • Dynamically access fields
  • Fetch variable-key metadata
  • Pair with $map or $objectToArray for dynamic access
  • $setField, $objectToArray, $getField, $mergeObjects, $literal

🧠 Notes

  • Avoids $[dot] notation for dynamic fields
  • Returns null if the field does not exist