Process Stock Adjustments (DML)
By the end of this lesson, you'll be able to:
- Increase stock when a shipment is received
- Decrease stock when an order ships out, without allowing it to go negative
- Explain why an adjustment reads the current value before writing a new one
Prerequisites: "Query Low-Stock Items"
Receiving a shipment (increasing stock)
public void receiveShipment(Id warehouseStockId, Decimal quantityReceived) {
Warehouse_Stock__c stock = [
SELECT Id, Quantity_On_Hand__c FROM Warehouse_Stock__c WHERE Id = :warehouseStockId
];
stock.Quantity_On_Hand__c += quantityReceived;
update stock;
}Reading the record first, then adding to its current value, is exactly Module 19's "Update" lesson pattern — += here is Module 4's compound assignment operator, doing real inventory work.
Shipping an order out (decreasing stock)
public Boolean shipOrder(Id warehouseStockId, Decimal quantityShipped) {
Warehouse_Stock__c stock = [
SELECT Id, Quantity_On_Hand__c FROM Warehouse_Stock__c WHERE Id = :warehouseStockId
];
if (stock.Quantity_On_Hand__c < quantityShipped) {
System.debug('Not enough stock to ship this quantity.');
return false;
}
stock.Quantity_On_Hand__c -= quantityShipped;
update stock;
return true;
}Checking Quantity_On_Hand__c < quantityShipped before subtracting prevents stock from ever going negative — a warehouse can't ship out more than it actually has, and this method returns false rather than silently creating an impossible negative quantity.
Why read-then-write, not a blind overwrite
// DON'T DO THIS
stock.Quantity_On_Hand__c = quantityReceived; // overwrites, doesn't addSetting the field directly to the shipment quantity would erase whatever stock was already there instead of adding to it — the read-then-modify-then-write pattern (query the current value, change it, update) is what makes an adjustment actually additive rather than destructive.
Exercise
Given a Warehouse_Stock__c already queried with Quantity_On_Hand__c, write code that adds 25 to its quantity and saves the change.
Show hint
stock.Quantity_On_Hand__c += 25; update stock;
Process Stock Adjustments (DML) Quiz
My Notes
Log in to keep private notes on this lesson.
Questions about this lesson
No questions yet — be the first to ask.
Log in to ask a question about this lesson.
Summary
A stock adjustment is never a blind overwrite — it always reads the current quantity first, then applies a change relative to it, guarding against an impossible negative result.