Workshop 201, Tutorial 5, Step 1, Apex

DeleteRestrictInvoice.cls

trigger DeleteRestrictInvoice on Invoice__c (before delete) {

  // create a list of Invoices in Trigger.oldMap along with their Line Items
  List<Invoice__c> invoices = [Select i.Name, (Select Name From Line_Items__r)
                                 From Invoice__c i 
                                Where i.Id IN :Trigger.oldMap.keySet()];

  // loop through the Invoices, attaching errors to those that have Line Items
  for (Invoice__c invoice : invoices) {
    if (!invoice.Line_Items__r.isEmpty()) {
      Trigger.oldMap.get(invoice.id).addError('Cannot delete Invoice with Line Items');
    }
  }
}

Thanks for the tip.

Merchandise is an required field. I realized that I can't access current DB by selecting first record from the Merchandise table in an Unite Test.

You must create a dummy object for testing, which it does make sense. Finally, I can complete my hands on tutorial.