$mergeObjects

The $mergeObjects operator merges multiple documents (objects) into a single document. Later fields overwrite earlier ones.

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📌 Syntax

{ "$mergeObjects": [ <doc1>, <doc2>, ... ] }

• Accepts an array of documents or document expressions
• Fields in later documents override those in earlier ones

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✅ Base Example – Merge Static Objects

📥 Input Document

{ "a": { "x": 1 }, "b": { "y": 2, "x": 3 } }

📌 Expression

{ "$mergeObjects": ["$a", "$b"] }

📤 Output

{ "x": 3, "y": 2 }

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🧱 Ecommerce Example – Combine Product Attributes

📌 Pipeline

[
  {
    "$project": {
      "product": "$name",
      "fullSpec": {
        "$mergeObjects": ["$specs.general", "$specs.technical"]
      }
    }
  }
]

📥 Input Document

{
  "name": "Tablet",
  "specs": {
    "general": { "brand": "XTab", "color": "Black" },
    "technical": { "ram": "8GB", "color": "Gray" }
  }
}

📤 Output

{
  "product": "Tablet",
  "fullSpec": {
    "brand": "XTab",
    "color": "Gray",
    "ram": "8GB"
  }
}

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🔧 Common Use Cases

• Merge attribute sets
• Combine multiple partial documents
• Override fields selectively

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🔗 Related Operators

• $arrayToObject, $objectToArray, $getField, $setField

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🧠 Notes

• If merging non-documents (e.g., null, arrays), behavior may vary
• Null values are ignored unless merged explicitly

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