Decrement only a single value in MongoDB?

To decrement only a single value in MongoDB, use the $inc operator with a negative value. This allows you to decrease a numeric field by a specified amount while targeting only one document.

Syntax

db.collection.update(
    { field: "matchValue" },
    { $inc: { numericField: -decrementValue } }
);

Create Sample Data

db.decrementingOperationDemo.insertMany([
    { "ProductName": "Product-1", "ProductPrice": 756 },
    { "ProductName": "Product-2", "ProductPrice": 890 },
    { "ProductName": "Product-3", "ProductPrice": 994 },
    { "ProductName": "Product-4", "ProductPrice": 1000 }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5cd7a8ae6d78f205348bc63c"),
        ObjectId("5cd7a8b86d78f205348bc63d"),
        ObjectId("5cd7a8c66d78f205348bc63e"),
        ObjectId("5cd7a8d06d78f205348bc63f")
    ]
}

Display All Documents

db.decrementingOperationDemo.find().pretty();

{
    "_id": ObjectId("5cd7a8ae6d78f205348bc63c"),
    "ProductName": "Product-1",
    "ProductPrice": 756
}
{
    "_id": ObjectId("5cd7a8b86d78f205348bc63d"),
    "ProductName": "Product-2",
    "ProductPrice": 890
}
{
    "_id": ObjectId("5cd7a8c66d78f205348bc63e"),
    "ProductName": "Product-3",
    "ProductPrice": 994
}
{
    "_id": ObjectId("5cd7a8d06d78f205348bc63f"),
    "ProductName": "Product-4",
    "ProductPrice": 1000
}

Decrement Single Value

Decrease Product-4's price by 10 units ?

db.decrementingOperationDemo.update(
    { _id: ObjectId("5cd7a8d06d78f205348bc63f"), ProductPrice: { $gt: 0 } },
    { $inc: { ProductPrice: -10 } }
);

WriteResult({ "nMatched": 1, "nUpserted": 0, "nModified": 1 })

Verify Result

db.decrementingOperationDemo.find().pretty();

{
    "_id": ObjectId("5cd7a8ae6d78f205348bc63c"),
    "ProductName": "Product-1",
    "ProductPrice": 756
}
{
    "_id": ObjectId("5cd7a8b86d78f205348bc63d"),
    "ProductName": "Product-2",
    "ProductPrice": 890
}
{
    "_id": ObjectId("5cd7a8c66d78f205348bc63e"),
    "ProductName": "Product-3",
    "ProductPrice": 994
}
{
    "_id": ObjectId("5cd7a8d06d78f205348bc63f"),
    "ProductName": "Product-4",
    "ProductPrice": 990
}

Key Points

• The $inc operator with negative values decrements numeric fields.
• Adding ProductPrice: { $gt: 0 } ensures the price is positive before decrementing.
• Only the matched document is modified; other documents remain unchanged.

Conclusion

Use $inc with negative values to decrement single field values in MongoDB. This operator provides precise control over numeric field modifications while maintaining data integrity.