I am working in an invoice form in Odoo, what i want is to display all the related details of a customer selected in the form from database.I have given an onchange context in the xml

field string="Student" name="partner_id"
                           on_change=" _get_joined_student_data(partner_id)" context="{'default_branch':branch,'default_course':course,'default_faculty':faculty,'search_default_customer':1, 'show_address': 1,'special_display_name': True,'default_time_slot':time_slot}"
                            options='{"always_reload": True}'
                            domain="[('customer', '=', True)]"/>

And the corresponding function definition for the function is given below

 def _get_joined_student_data(self,partner_id):
    print "function"
    datas={}
    print self
    if partner_id:
        print partner_id
        self._cr.execute("SELECT c.id,c.fee,b.id,t.id,f.id FROM custom_joinform cj LEFT JOIN course_custom c on cj.course=c.id "+
           "LEFT JOIN branch_custom b on cj.branch=b.id LEFT JOIN custom_timeslot t on cj.time_slot=t.id LEFT JOIN custom_faculty f on cj.faculty=f.id LEFT JOIN res_partner r on r.id=cj.name where r.id="+str(partner_id)+" and function='Student'")
           #print "SELECT c.name,c.fee,b.name,t.name,f.name FROM custom_joinform cj LEFT JOIN course_custom c on cj.course=c.id LEFT JOIN branch_custom b on cj.branch=b.id LEFT JOIN custom_timeslot t on cj.time_slot=t.id LEFT JOIN custom_faculty f on cj.faculty=f.id LEFT JOIN res_partner r on r.id=cj.name where r.id="+str(self.name.id)+" and function='Student'"
        for res in self.env.cr.fetchall():
            datas={
                   'course':res[0],
                   'course_fee':res[1],
                   'branch':res[2],
                   'time_slot':res[3],
                   'faculty':res[4]
                  }
        print datas
        self.update(datas)

And the print statements inside the function is printing correctly. But nothing happens in the form after changing the selection.Can anyone help me to solve this issue.