<h2>Full example</h2>
<pre><code>// source: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_shopping_cart_example_code.htm
trigger calculate on Item__c (after insert, after update, after delete) {
// Use a map because it doesn't allow duplicate values
Map<ID, Shipping_Invoice__C> updateMap = new Map<ID, Shipping_Invoice__C>();
// Set this integer to -1 if we are deleting
Integer subtract ;
// Populate the list of items based on trigger type
List<Item__c> itemList;
if(trigger.isInsert || trigger.isUpdate){
itemList = Trigger.new;
subtract = 1;
}
else if(trigger.isDelete)
{
// Note -- there is no trigger.new in delete
itemList = trigger.old;
subtract = -1;
}
// Access all the information we need in a single query
// rather than querying when we need it.
// This is a best practice for bulkifying requests
set<Id> AllItems = new set<id>();
for(item__c i :itemList){
// Assert numbers are not negative.
// None of the fields would make sense with a negative value
System.assert(i.quantity__c > 0, 'Quantity must be positive');
System.assert(i.weight__c >= 0, 'Weight must be non-negative');
System.assert(i.price__c >= 0, 'Price must be non-negative');
// If there is a duplicate Id, it won't get added to a set
AllItems.add(i.Shipping_Invoice__C);
}
// Accessing all shipping invoices associated with the items in the trigger
List<Shipping_Invoice__C> AllShippingInvoices = [SELECT Id, ShippingDiscount__c,
SubTotal__c, TotalWeight__c, Tax__c, GrandTotal__c
FROM Shipping_Invoice__C WHERE Id IN :AllItems];
// Take the list we just populated and put it into a Map.
// This will make it easier to look up a shipping invoice
// because you must iterate a list, but you can use lookup for a map,
Map<ID, Shipping_Invoice__C> SIMap = new Map<ID, Shipping_Invoice__C>();
for(Shipping_Invoice__C sc : AllShippingInvoices)
{
SIMap.put(sc.id, sc);
}
// Process the list of items
if(Trigger.isUpdate)
{
// Treat updates like a removal of the old item and addition of the
// revised item rather than figuring out the differences of each field
// and acting accordingly.
// Note updates have both trigger.new and trigger.old
for(Integer x = 0; x < Trigger.old.size(); x++)
{
Shipping_Invoice__C myOrder;
myOrder = SIMap.get(trigger.old[x].Shipping_Invoice__C);
// Decrement the previous value from the subtotal and weight.
myOrder.SubTotal__c -= (trigger.old[x].price__c *
trigger.old[x].quantity__c);
myOrder.TotalWeight__c -= (trigger.old[x].weight__c *
trigger.old[x].quantity__c);
// Increment the new subtotal and weight.
myOrder.SubTotal__c += (trigger.new[x].price__c *
trigger.new[x].quantity__c);
myOrder.TotalWeight__c += (trigger.new[x].weight__c *
trigger.new[x].quantity__c);
}
for(Shipping_Invoice__C myOrder : AllShippingInvoices)
{
// Set tax rate to 9.25% Please note, this is a simple example.
// Generally, you would never hard code values.
// Leveraging Custom Settings for tax rates is a best practice.
// See Custom Settings in the Apex Developer Guide
// for more information.
myOrder.Tax__c = myOrder.Subtotal__c * .0925;
// Reset the shipping discount
myOrder.ShippingDiscount__c = 0;
// Set shipping rate to 75 cents per pound.
// Generally, you would never hard code values.
// Leveraging Custom Settings for the shipping rate is a best practice.
// See Custom Settings in the Apex Developer Guide
// for more information.
myOrder.Shipping__c = (myOrder.totalWeight__c * .75);
myOrder.GrandTotal__c = myOrder.SubTotal__c + myOrder.tax__c +
myOrder.Shipping__c;
updateMap.put(myOrder.id, myOrder);
}
}
else
{
for(Item__c itemToProcess : itemList)
{
Shipping_Invoice__C myOrder;
// Look up the correct shipping invoice from the ones we got earlier
myOrder = SIMap.get(itemToProcess.Shipping_Invoice__C);
myOrder.SubTotal__c += (itemToProcess.price__c *
itemToProcess.quantity__c * subtract);
myOrder.TotalWeight__c += (itemToProcess.weight__c *
itemToProcess.quantity__c * subtract);
}
for(Shipping_Invoice__C myOrder : AllShippingInvoices)
{
// Set tax rate to 9.25% Please note, this is a simple example.
// Generally, you would never hard code values.
// Leveraging Custom Settings for tax rates is a best practice.
// See Custom Settings in the Apex Developer Guide
// for more information.
myOrder.Tax__c = myOrder.Subtotal__c * .0925;
// Reset shipping discount
myOrder.ShippingDiscount__c = 0;
// Set shipping rate to 75 cents per pound.
// Generally, you would never hard code values.
// Leveraging Custom Settings for the shipping rate is a best practice.
// See Custom Settings in the Apex Developer Guide
// for more information.
myOrder.Shipping__c = (myOrder.totalWeight__c * .75);
myOrder.GrandTotal__c = myOrder.SubTotal__c + myOrder.tax__c +
myOrder.Shipping__c;
updateMap.put(myOrder.id, myOrder);
}
}
// Only use one DML update at the end.
// This minimizes the number of DML requests generated from this trigger.
update updateMap.values();
}</code></pre>
