$subtract

The $subtract operator subtracts one number (or date) from another.

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📌 Syntax

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

<expression1> is the value to subtract from.
<expression2> is the value to subtract.

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✅ Base Example – Subtract Discount from Price

📥 Input Document

{ "price": 100, "discount": 25 }

📌 Expression

{ "$subtract": ["$price", "$discount"] }

📤 Output

75

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🧱 Deep Nested Example – Compute Remaining Balance per Item

📌 Pipeline

[
  { "$unwind": "$items" },
  {
    "$project": {
      "product": "$items.name",
      "due": {
        "$subtract": ["$items.total", "$items.paid"]
      }
    }
  }
]

📥 Input Document

{
  "orderId": 2,
  "items": [
    { "name": "Monitor", "total": 200, "paid": 150 },
    { "name": "Keyboard", "total": 100, "paid": 100 }
  ]
}

📤 Output Documents

[
  { "product": "Monitor", "due": 50 },
  { "product": "Keyboard", "due": 0 }
]

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🔧 Common Use Cases

• Calculating remaining balances
• Date differences (with ISODate types)
• Price reductions and change tracking

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🔗 Related

• $add, $multiply, $divide
• $project, $group, $set

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