$multiply

The $multiply operator multiplies numbers together.

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📌 Syntax

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

Each expression must resolve to a numeric value.

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✅ Base Example – Multiply Price by Quantity

📥 Input Document

{ "price": 20, "qty": 3 }

📌 Expression

{ "$multiply": ["$price", "$qty"] }

📤 Output

60

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🧱 Deep Nested Example – Compute Line Totals in Order

📌 Pipeline

[
  { "$unwind": "$items" },
  {
    "$project": {
      "product": "$items.name",
      "lineTotal": {
        "$multiply": ["$items.price", "$items.quantity"]
      }
    }
  }
]

📥 Input Document

{
  "orderId": 1001,
  "items": [
    { "name": "Pen", "price": 2, "quantity": 5 },
    { "name": "Notebook", "price": 3, "quantity": 2 }
  ]
}

📤 Output Documents

[
  { "product": "Pen", "lineTotal": 10 },
  { "product": "Notebook", "lineTotal": 6 }
]

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🔧 Common Use Cases

• Calculating total cost (price * qty)
• Tax/value scaling
• Aggregated computations in $group

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🔗 Related

• $add, $subtract, $divide, $mod
• $group, $project, $set

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