How to subtract values (TotalPrice – Discount) from document field values in MongoDB?

To subtract values from document field values in MongoDB, use the $subtract operator within the aggregation pipeline. This operator performs arithmetic subtraction between numeric field values or expressions.

Syntax

db.collection.aggregate([
    {
        $project: {
            newField: { 
                $subtract: ["$field1", "$field2"] 
            }
        }
    }
]);

Sample Data

db.demo599.insertMany([
    {"TotalPrice": 250, "DiscountPrice": 35},
    {"TotalPrice": 400, "DiscountPrice": 10},
    {"TotalPrice": 1550, "DiscountPrice": 50}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e948192f5f1e70e134e2696"),
        ObjectId("5e948199f5f1e70e134e2697"),
        ObjectId("5e9481a0f5f1e70e134e2698")
    ]
}

Display Sample Data

db.demo599.find();

{ "_id": ObjectId("5e948192f5f1e70e134e2696"), "TotalPrice": 250, "DiscountPrice": 35 }
{ "_id": ObjectId("5e948199f5f1e70e134e2697"), "TotalPrice": 400, "DiscountPrice": 10 }
{ "_id": ObjectId("5e9481a0f5f1e70e134e2698"), "TotalPrice": 1550, "DiscountPrice": 50 }

Calculate Actual Price (TotalPrice - DiscountPrice)

db.demo599.aggregate([
    {
        $project: {
            ActualPrice: { 
                $subtract: ["$TotalPrice", "$DiscountPrice"] 
            }
        }
    }
]);

{ "_id": ObjectId("5e948192f5f1e70e134e2696"), "ActualPrice": 215 }
{ "_id": ObjectId("5e948199f5f1e70e134e2697"), "ActualPrice": 390 }
{ "_id": ObjectId("5e9481a0f5f1e70e134e2698"), "ActualPrice": 1500 }

Key Points

• $subtract takes an array with exactly two numeric expressions: [$minuend, $subtrahend]
• Field references must be prefixed with $ (e.g., "$TotalPrice")
• The operator only projects the calculated field by default; use $$ROOT to preserve original fields

Conclusion

The $subtract operator in MongoDB's aggregation pipeline efficiently performs arithmetic subtraction between document fields. Use it within $project to create computed fields based on existing numeric values.