I'm new to Apex/Visforce and I'm having trouble wrapping my brain around test methods and getting them to work. I currently have a method that is giving me the following error and I can't figure out why:

System.SObjectException: Field is not writeable: Merchant_Analysis__c.Opportunity__c

 It refers to the bolded line below:

public PageReference save()
{
analysis.Opportunity__c = oppID;
analysis.recordtypeid = recType;
insert analysis;

PageReference analysisPage = new PageReference('/' + analysis.id);
analysisPage.setRedirect(true);

return analysisPage;
}

static testMethod void testSave()
{
analysisController controller = new analysisController();
String recType = '012400000005QI0AAM';

Opportunity o = new Opportunity(name='test opp',stagename='test stage',closedate=System.today());
insert o;

Merchant_Analysis__c a = new Merchant_Analysis__c();
a.Opportunity__c = o.id;
a.recordtypeid=recType;
insert a;

PageReference testPage = new PageReference('/' + a.id);
testPage.setRedirect(true);

controller.oppID = o.id;
controller.recType = recType;

controller.analysis = a;

String ref0 = testPage.getUrl();
String ref1 = controller.save().getUrl();

System.assertEquals(ref0,ref1);
}

If someone could help troubleshoot this issue with me that would be awesome.

Thanks,

Jonathan