trying to write a test for an apex trigger - triggers

Question

I'm working on a trigger/test class and i can't figure out how to get the test class to work. I know that i need to update my opportunity to use the trigger but i'm not sure how to and then how to verify that my trigger is working.
Trigger:
trigger add_primary_advisor on Opportunity(before update) {
for(Opportunity o: Trigger.new){
if (o.IsClosed && !Trigger.oldMap.get(o.id).IsClosed) {
OpportunityContactRole contactRole =
[select ContactID from OpportunityContactRole where IsPrimary = true and OpportunityId = :o.id];
if (contactRole != null) {
o.Primary_Advisor__c=contactRole.ContactID;
}
}
}
}
Test class:
#isTest
private class primary_advisor_test {
static testMethod void primary_advisor(){
Opportunity opp = new Opportunity(Name='test opp', StageName='stage', Probability = 95, CloseDate=system.today());
insert opp;
update opp;
}
}

Answer 1

Before going into the solution for the test class I would like to point out the trigger is not builkified as you have a SOQL query inside you for loop which is not a best practice.
I am not aware of the exact functionality of the opportunityContactRole object, I am just assuming that it is a object which will hold the contact id and the opportunityID, more or less like a junction object.
#isTest
private class primary_advisor_test {
static testMethod void primary_advisor(){
//Create a contact that will be added to the opportunityCOntactRole.
contact con = new contact(name='testCon');// add all the required field as per your org settings
insert Con;
Opportunity opp = new Opportunity(Name='test opp', StageName='stage', Probability = 95, CloseDate=system.today());
insert opp;
//Create the opporunityContactRole.
opportunityCOntactRole oppCOn = new new opportunityCOntactRole(OpportunityId=opp.id, contactId= con.Id, isPrimary=true);
insert oppCon;
//update the opportunity so that it is closed and enters the if conditon in your trigger.
opp.stageName='Closed';
update opp;
}
}

Answer 2

The problem is you didn't changed any fields before calling update try this
opp.Probability = 90;
update opp;