$replaceRoot

The $replaceRoot stage replaces the entire input document with a specified subdocument.

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📌 Syntax

{ "$replaceRoot": { "newRoot": <expression> } }

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✅ Base Example – Replace Root with Embedded Customer Object

📥 Input Document

{
  "_id": 1,
  "orderId": 22,
  "customer": {
    "name": "Alice",
    "email": "alice@example.com"
  },
  "items": [{ "name": "Book" }]
}

📌 Stage

{ "$replaceRoot": { "newRoot": "$customer" } }

📤 Output Document

{
  "name": "Alice",
  "email": "alice@example.com"
}

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🔧 Common Operators

• $mergeObjects, $ifNull, $project

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🔗 Related

• $replaceWith, $project, $unwind

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