Used AssociatedUser to save agent, referrer & admissionCounselor of
students.
created a transient property long aoId in AssociatedUser class to hold
the id of the AdmissionOfficer
in the form I use path="aoId" in the form select field
when user select admissionCounselor from the dropDown menu, set the
admissionCounselors.id in aoId
in the controller get aoId from the associatedUser ModelAttribute and 
pass aoId to associatedUser.setAdmissionOfficer