Test Class for Trigger before insert before update

Question

I am very new to APEX and I have written the following trigger that updates the due date on a case based upon an SLA value stored in another object once the user fills a value in a lookup field

trigger SetCaseSLA on Case (before insert, before update) {

    for (Case ca : Trigger.new) {
            if ((ca.CaseType__c != NULL) && (ca.DueDate__c == NULL)) {

            id CaseTypeId = ca.CaseType__c;

            List<CaseType__c> results = [select SLA__c from caseType_c where id = :CaseTypeId];
            Integer mySLA = Integer.valueOf(results[0].get('SLA__c'));


            ca.DueDate__c = BusinessHours.add(ca.BusinessHoursId, System.now(), mySLA * 60 * 60 * 1000L);

            }
      }
 }

Then I have written the following Apex Test Class however it would appear that the DueDate is always null and the trigger is not firing is there something wrong with the setup of the test class causing the trigger not to fire?

@isTest
private class TestSetDueDate {
    @isTest static void TestDueDate() {

        CaseType__c csecat = new CaseType__c();
        csecat.CaseTypeName__c = 'My Type Test';
        csecat.SLA__c = 2;

        insert csecat;

        Case cse = new Case();
        cse.Origin='Email';
        cse.BusinessHoursID = '01m3C00000008d2QAA';

        Test.startTest();

        insert cse;

        cse.CaseType__c = csecat.id;

        update cse;

        Test.stopTest();

        System.debug(csecat.SLA__c);
        System.debug(cse.DueDate__c);
        System.debug(cse.CaseType__c);


        System.assertEquals(cse.DueDate__c , BusinessHours.add(cse.BusinessHoursId, system.now(), 2 * 60 * 60 * 1000L));
    }

}

Answer 1

As in all other Apex functionality, you must query fields that you wish to update on your sObject variables. The only field that is automatically updated for you is Id after an insert DML.

After Test.stopTest(), you must do

cse = [SELECT CaseType__c, DueDate__c FROM Case WHERE Id = :cse.Id];

Only then can you perform assertions to validate your code's behavior.