Intermediate 30 min read

Track Inventory Levels

By the end of this lesson, you'll be able to:

Write an InventoryService class with a method to record initial stock
Query total stock for a product summed across every warehouse
Explain how a relationship query reaches from product to its stock records

Prerequisites: "Design the Stock Model"

Recording initial stock

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;
    }
}

This is Module 22's service-class pattern, applied to a new object — InventoryService is the single entry point for every inventory operation, exactly like LibraryService was for library operations.

Summing stock across every warehouse

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');
}

This is Module 20's SUM() aggregate function, put to real use — instead of querying every Warehouse_Stock__c record and adding them up in an Apex loop, the database computes the total directly.

Listing stock by warehouse

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
    ];
}

This answers a different, complementary question from getTotalStock — not "how much in total?" but "how much where?" — both useful views over the exact same underlying Warehouse_Stock__c records.

Exercise

Add a getWarehouseCount(Id productId) method returning how many distinct warehouses currently stock a product, using COUNT().

Show hint

SELECT COUNT() FROM Warehouse_Stock__c WHERE Product__c = :productId

APEX

Track Inventory Levels Quiz

1. Why use SUM(Quantity_On_Hand__c) in SOQL rather than looping over records and adding in Apex?

The database computes the total directly, avoiding an unnecessary loop over every record Apex cannot perform addition SUM() is required by Salesforce for every query It returns a List instead of a single value

My Notes

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Summary

This lesson builds InventoryService's foundation — recording initial stock, and answering "how much of this product do we have, total, across every warehouse?"

← Design the Stock Model Query Low-Stock Items →