title: Pair of values Unique in Junctiontable
body: <p>can we make pair of values unique in Junction Table? two tables (Students and classrooms) have many to many relationship. I have created a junction table which contains Studentid and Classroom id only. But i do not want same pair to be enetered twice. Forexample (student id:33 Classroom id:5).
  Please help!!!</p>