I am trying to populate a wrapper class field in my test method.  I attempted the syntax used in the functional controller extension which populates the key field.  Only I'm hard coding an associated field. The object is a custom junction object for the 1 to many relationship.

My error is:  Variable does not exist: Opportunity__c at line 64 column 6

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The class is:

public class OpportunityLineItem {

public Integer lineNumber{ get; set; }
public Boolean isDeleted{ get; set; }
public tripOpportunityAssociation__c opportunity { get; set;}

public opportunityLineItem(){
opportunity = new tripOpportunityAssociation__c();
lineNumber = 0;
isDeleted = false;
}

public OpportunityLineItem(Integer pLineNumber, tripOpportunityAssociation__c pOpportunityItem){
opportunity = pOpportunityItem;
lineNumber = pLineNumber;
isDeleted = false;
}

The Controller extension population of the key field is:

 PageReference addAttendee;

// Populate new opportunity items with report id.

for ( TripOpportunityAssociation__c item : opportunityItemsToUpsert ){
if (item.Trip_Opportunity__c == null){
item.Trip_Opportunity__c = tripReport.id;

}
}

---------------

The test method syntax is:

@isTest private class TestTripRptExt3 {
public TestTripRptExt3() {

}
//Public attributes.
public ApexPages.StandardController controller;

public Boolean CancelConfirm = false;
//Private attributes.
private Trip_Report__c tripReport;
private String reportItemsMessage;

private Integer attendeeLineCount = 0;
private Integer opportunityLineCount = 0;

//Controller extension constructor.
public TestTripRptExt3(ApexPages.StandardController stdController) {


static testMethod void testTripRptExt3() {

// controller.setUnique_Name & basic fields(
controller.tripReport.Unique_Name__c = 'Test Method Name';
controller.tripReport.Date_of_Meeting__c = date.parse('07/13/2012');
controller.tripReport.Meeting_Notes__c = 'Test Method Note';

PageReference addOpportunity;
for(OpportunityLineItem item : controller.opportunityLineItemList) {
item.Opportunity__c = 'Great Opportunity';
}

}

Any ideas on this?  Thanks