Track Inventory Levels

public class InventoryService {
    public Warehouse_Stock__c receiveStock(Id productId, String warehouseName, Decimal quantity) {
        Warehouse_Stock__c stock = new Warehouse_Stock__c(
            Product__c = productId,
            Warehouse_Name__c = warehouseName,
            Quantity_On_Hand__c = quantity
        );
        insert stock;
        return stock;
    }
}

1. public class InventoryService { — this is Module 22's service-class pattern, applied to a new object.
2. public Warehouse_Stock__c receiveStock(Id productId, String warehouseName, Decimal quantity) { — the single entry point for recording initial stock.
3. Warehouse_Stock__c stock = new Warehouse_Stock__c( — builds the record.
4. Product__c = productId, — links to the product.
5. Warehouse_Name__c = warehouseName, — names the warehouse.
6. Quantity_On_Hand__c = quantity — the starting quantity.
7. ); — closes the constructor call.
8. insert stock; — saves it — InventoryService is the single entry point for every inventory operation, exactly like LibraryService was for library operations.
9. return stock; — hands back the saved record.
10. } — closes the method.
11. } — closes the class.

public Decimal getTotalStock(Id productId) {
    AggregateResult result = [
        SELECT SUM(Quantity_On_Hand__c) totalQty
        FROM Warehouse_Stock__c
        WHERE Product__c = :productId
    ];
    return (Decimal) result.get('totalQty');
}

1. public Decimal getTotalStock(Id productId) { — answers "how much of this product do we have, total?"
2. AggregateResult result = [ — opens the query.
3. SELECT SUM(Quantity_On_Hand__c) totalQty — this is Module 20's SUM() aggregate function, put to real use — the database computes the total directly.
4. FROM Warehouse_Stock__c — the object.
5. WHERE Product__c = :productId — scoped to one product.
6. ]; — closes the query.
7. return (Decimal) result.get('totalQty'); — reads the computed value via its alias, instead of querying every Warehouse_Stock__c record and adding them up in an Apex loop.
8. } — closes the method.

public List<Warehouse_Stock__c> getStockByWarehouse(Id productId) {
    return [
        SELECT Id, Warehouse_Name__c, Quantity_On_Hand__c
        FROM Warehouse_Stock__c
        WHERE Product__c = :productId
        ORDER BY Warehouse_Name__c
    ];
}

1. public List<Warehouse_Stock__c> getStockByWarehouse(Id productId) { — a different, complementary question from getTotalStock — not "how much in total?" but "how much where?"
2. return [ — opens the query.
3. SELECT Id, Warehouse_Name__c, Quantity_On_Hand__c — the fields needed.
4. FROM Warehouse_Stock__c — the object.
5. WHERE Product__c = :productId — scoped to one product.
6. ORDER BY Warehouse_Name__c — sorted alphabetically by warehouse.
7. ]; — closes the query — both useful views over the exact same underlying Warehouse_Stock__c records.
8. } — closes the method.