code: def form_valid_bridge(self, form, field, model, related_field, error_message):
          
          # get instance of selected external object
          external = form.cleaned_data[field]
          related_object = get_external_model(model).objects.filter(pk=external.pk).first()
          if related_object:
              # form valid
              try:
                  with transaction.atomic():
                      result = super(GenCreateBridge, self).form_valid(form)
                      if get_external_model(model).objects.filter(**{
                              "pk": external.pk,
                              "{}__isnull".format(related_field): False
                              }).exists():
                          errors = form._errors.setdefault(field, ErrorList())
                          errors.append(error_message[0])
                          raise CodenerixException()
                      else:
                          setattr(related_object, related_field, form.instance)
                          related_object.save()
                          if hasattr(self.object, 'CDNX_refresh_permissions_CDNX'):
                              self.object.CDNX_refresh_permissions_CDNX()
                  # Return the object already saved properly
                  answer = HttpResponseRedirect(self.get_success_url())
              except CodenerixException:
                  answer = super(GenCreateBridge, self).form_invalid(form)
              except IOError as e:
                  errors = form._errors.setdefault("codenerix_external_field", ErrorList())
                  errors.append(e)
                  answer = super(GenCreateBridge, self).form_invalid(form)
          else:
              errors = form._errors.setdefault(field, ErrorList())
              errors.append(error_message[1])
              answer = super(GenCreateBridge, self).form_invalid(form)
  
          # Return
          return answer
docs: call from form_valid.
          @form: it is form of form_valid (type: form)
          @field: name of the form field referring to the brigde (type: string)
          @model: form's model (type class)
          @related_field: name of the related_name of the brigde (type: string)
          @error_message: list of message errors (type list)
              two items, example:
              ["The selected entry is already a storage, select another entry!",
              "The selected entry is not available anymore, please, try again!"]