Unable to cover these lines in my test Apex class

I am unable to cover the below Salesforce Apex method. Can any one please help me in writing a test class?

public class Red{

    private final SPA__Document__c QD;
    public quoteDocRed(ApexPages.StandardController controller)
    {
        this.QD = (SPA__Document__c ) controller.getRecord();
    }

    public PageReference pageReferenceMethod()
    {
        String a = [select SB__Agent__c from SPA__Document__c where id =: QD.id].SB__Agent__c;
        return new PageReference('/' + a);
    }

}