code: def clean(self):
          
  
          data = self.cleaned_data
          formats = Format.objects.filter(name=data['name'])
          if self.instance:
              formats = formats.exclude(pk=self.instance.pk)
  
          exists_sites = []
          for f in formats:
              for s in f.sites.all():
                  if s in data['sites']:
                      exists_sites.append(s.__unicode__())
  
          if len(exists_sites):
              raise ValidationError(ugettext("Format with this name exists for site(s): %s" % ", ".join(exists_sites)))
  
          return data
docs: Check format name uniqueness for sites
  
          :return: cleaned_data