How do I test for a trigger that inserts a calculated value after insert/update of record?

Question

I'm trying to test for a trigger that insert a calculated duration of Event after insert/update of A Event record. For example, the trigger fills an integer representing hours, calculated from StartDateTime and EndDateTime of the subject Event record, to DurationInHours__c field.

The problem is that DurationInHours__c is null when System.assertEquals is executed. Error message says 'System.AssertException: Assertion Failed: Expected: 1, Actual: null'. Note that event duration is set to be 1 hour.

Please help me out.

Test code:

@isTest
public class TestUpdateDuration {
    @isTest static void TestUpdateDuration() {
        // Test data setup
        // Create an event with 1 hour duration
        User myUser =[select id from User where Email = 'foobar@mail.com'];

        Datetime myStart = Datetime.newInstance(2019, 9,17, 7,0,0);
        Datetime myEnd = Datetime.newInstance(2019, 9,17, 8,0,0);
        Event ev = new Event(
                             StartDateTime = myStart,
                             EndDateTime = myEnd 
                            );
        insert ev;

        // Verify 
        // In this case '1' should be added to DurationInHours__c field by the trigger,
        // so verify if it is correct
        System.assertEquals(1, ev.DurationInHours__c);
    }    
}

Trigger: Event record will have an event duration value in DurationInHours__c after update/insert. I manually check that a value is inserted and it was ok.

trigger updateDuration on Event (after update, after insert) {
    List<Event> listEvents = new List<Event>();

    for ( Event e : Trigger.new ) {
        Event updateE = [select id, EndDateTime, StartDateTime, DurationInHours__c, DurationInMinutes, IsAllDayEvent from Event where id = :e.id];

        // How many days will that event last?
        Long msecStart = updateE.StartDateTime.getTime();
        Long msecEnd = updateE.EndDateTime.getTime();

        Long msecDuration = msecEnd - msecStart;
        Long elapsedHours = 0;
        if(updateE.IsAllDayEvent) {
            elapsedHours = 8;
        } else {
            elapsedHours = ((msecDuration / 1000) / 60) / 60;
        }

        if(updateE.DurationInHours__c != elapsedHours) {
           updateE.DurationInHours__c = elapsedHours;
           listEvents.add(updateE);
        }    
    }
    update listEvents;
}

Answer 1

Welcome to Salesforce StackExchange.

the test method creates an Event and inserts it into the database temporarily. So for retrieving the other value you should write the SOQL on Event to get the value of other Feild.

Please Read Apex Test Class Documentation.

So changes in your code need to do.

System.assertEquals(1,[Select Id, DurationInHours__c From Event Where Id = : 'eventId'].DurationInHours__c);

Important Tips

• Please use @testSetup method to create test data, and get the data in the test method using SOQL. @testSetup Document.
• I seeing you are using SOQL inside for loop in your trigger, which is an impact with large data means trigger are not bulkify, due to governer limits. Please see Test bulky trigger Document