Value is not valid while using more complex hql

Question

I'm writing Hibernate JSF application and I currently have one problem. I try to make in xhtml dropdown to select id of foreign key.

xhtml file:

<div class="form-group">
     <h:outputLabel for="student" class="control-label col-sm-2">Student</h:outputLabel>  
     <div class="col-sm-6">
           <h:selectOneMenu id="student" class="form-control" value="#{studentGroupID.studentID}" required="true">
           <f:selectItems value="#{data.showStudentsOfFaculty(studentGroupID.groupID)}" var="user" itemValue="#{user.studentID}" itemLabel="#{user.toString()}" />
           </h:selectOneMenu>
           <h:message for="student" style="color:red" />
     </div>
</div>

data.showStudentsOfFaculty():

public ArrayList<Student> showStudentsOfFaculty(int groupId) {
    session = helper.getSessionFactory().openSession();
    String hql = "select s.studentID, s.name, s.surname, s.address, s.city, s.emailAddress from FacultyStudent ws, Student s, Group g, Subject p "+
            "where ws.facultyStudentID.studentID=s.studentID and g.subjectID=p.subjectID and p.facultyID=ws.facultyStudentID.facultyID and g.groupID="+groupId;
    Iterator query = session.createQuery(hql).list().iterator();

    ArrayList<Student> students = new ArrayList<>();

    while ( query.hasNext() ) {
        Object[] results = (Object[]) query.next();
        Student st = new Student((String)results[1],(String)results[2],(String)results[3],(String)results[4],(String)results[5]);
        st.setStudentID((Integer)results[0]);
        students.add(st);   
    }
    session.close();
    return students;
}

Every time I try to sumbit form I get error:

addStudent:student: Validation Error: Value is not valid 

When I use other function data.showStudents() intead of one above form works perfectly but it shows too much data.

Data.showStudents():

    public List<Student> showStudents() {
    session = helper.getSessionFactory().openSession();
    String hql = "FROM Student S";
    Query query = session.createQuery(hql);
    List<Student> results = (List<Student>) query.list();
    session.close();
    return results;
}