$add

The $add operator adds numbers together or concatenates dates and numeric expressions.

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📌 Syntax

{ "$add": [ <expression1>, <expression2>, ... ] }

Each expression should resolve to a number or date.

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✅ Base Example – Add Two Fields

📥 Input Document

{ "price": 100, "tax": 20 }

📌 Expression

{ "$add": ["$price", "$tax"] }

📤 Output

120

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🧱 Deep Nested Example – Compute Total for Each Item in an Order

📌 Pipeline

[
  { "$unwind": "$items" },
  {
    "$project": {
      "product": "$items.name",
      "totalCost": { "$add": ["$items.price", "$items.tax"] }
    }
  }
]

📥 Input Document

{
  "orderId": 1,
  "items": [
    { "name": "Laptop", "price": 1000, "tax": 130 },
    { "name": "Mouse", "price": 50, "tax": 5 }
  ]
}

📤 Output Documents

[
  { "product": "Laptop", "totalCost": 1130 },
  { "product": "Mouse", "totalCost": 55 }
]

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🔧 Common Use Cases

• Calculating final prices
• Summing quantities
• Adding durations to timestamps

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🔗 Related

• $subtract, $multiply, $divide – arithmetic family
• $project, $group – stages where $add is commonly used

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