code: def formfield_for_dbfield(self, db_field, **kwargs):
          ''' Offer only gradings that are not used by other schemes, which means they are used by this scheme or not at all.'''
          if db_field.name == "gradings":
              request=kwargs['request']
              try:
                  #TODO:  MockRequst object from unit test does not contain path information, so an exception occurs during test.
                  #       Find a test-suite compatible solution here.
                  obj=resolve(request.path).args[0]
                  filterexpr=Q(schemes=obj) | Q(schemes=None)
                  kwargs['queryset'] = Grading.objects.filter(filterexpr).distinct()
              except:
                  pass
          return super(GradingSchemeAdmin, self).formfield_for_dbfield(db_field, **kwargsf formfield_for_dbfield(self, db_field, **kwargs):
          ''' Offer only gradings that are not used by other schemes, which means they are used by this scheme or not at all.'''
          if db_field.name == "gradings":
              request=kwargs['request']
              try:
                  #TODO:  MockRequst object from unit test does not contain path information, so an exception occurs during test.
                  #       Find a test-suite compatible solution here.
                  obj=resolve(request.path).args[0]
                  filterexpr=Q(schemes=obj) | Q(schemes=None)
                  kwargs['queryset'] = Grading.objects.filter(filterexpr).distinct()
              except:
                  pass
          return super(GradingSchemeAdmin, self).formfield_for_dbfield(db_field, **kwargs)
docs: Offer only gradings that are not used by other schemes, which means they are used by this scheme or not at all.