Good evening, 

need help getting the proper test coverage on trigger, and one issue i'm having with the test class is the field that i'm actually trying to test.  the trigger is to populate an Event record custom field ('Fortune_1000_Ranking__c') from a field pulled from the related Contact ('Fortune_1000_2013_Ranking__c'), which is actually a forrmula field pulled from the same field on the Account record (master-detail).  The trigger works fine:

trigger UpdateEventFortune1000 on Event (before insert, before update) {
Set<Id>CIds = new Set<Id>();for (Event E:trigger.new)
{String wId = e.WhoId; If(wID!=null && wId.startsWith('003')&& !cIds.contains(e.WhoId))
{cIds.add(e.WhoId);}}
    
    List<Contact> EventC = [Select Id, Fortune_1000_2013_Ranking__c from Contact where
                            Id in :cIds];
    
    Map<Id, Contact> CMap = new Map<Id, Contact>(); for (Contact c: EventC)
        
    {CMap.put(c.Id,c);}
    
    for (Event e:trigger.new)
    {String wId = e.WhoId; if (wId!=null && wId.startswith('003'))
    {Contact thisC = cMap.get(e.WhoId);
    if(thisC!=null)
    {e.Fortune_1000_Ranking__c = thisC.Fortune_1000_2013_Ranking__c;}}}
 
}

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The test class is throwing an error on the Fortune_1000_2013_Ranking__c line below due to the field is unwritable (value is pulled from the Account record)...any help would be appreciated!!

@isTest
private class UpdateEventFortune1000testclass {
    
static testMethod void UpdateEventFortune1000 () {
List<Contact>Contact = new List<Contact> {new Contact (
    firstname = 'test',
    lastname = 'testtest',
    Fortune_1000_2013_Ranking__c = 1,
    phone = '(555) 555-5555') }; 
    insert Contact;
    
List<Event> e = new List<Event> {new Event(
    WhoID = Contact[0].id, 
    subject = 'test trigger',
    Type = 'Meeting')};
    
    insert e;
    
List<Event> Eventstoupdate = New List<event>{[select id from event where id in :e]};
    for (event eok:eventstoupdate)
    eok.Fortune_1000_Ranking__c=null;
    
    update eventstoupdate;
    
Contact[] c = [select id,Fortune_1000_2013_Ranking__c from Contact where id in :Contact];
    System.assertEquals(1,Contact[0].Fortune_1000_2013_Ranking__c);
    
}
}