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)
            one item, example:
            ["The selected entry is not available anymore, please, try again!", ]

def form_valid_bridge(self, form, field, model, related_field, error_message):
        """
        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)
            one item, example:
            ["The selected entry is not available anymore, please, try again!", ]
        """
        # get instance of selected external object
        external = form.cleaned_data[field]
        object_edit = model.objects.get(pk=form.instance.pk)
        if object_edit.external == external:
            answer = super(GenUpdateBridge, self).form_valid(form)
        else:
            related_object = get_external_model(model).objects.filter(**{
                "pk": external.pk,
                "{}__isnull".format(related_field): False
            }).exclude(**{
                "{}".format(related_field): form.instance,
            }).first()
            if not related_object:
                # form valid
                try:
                    with transaction.atomic():
                        result = super(GenUpdateBridge, self).form_valid(form)
                        field = object_edit.external
                        setattr(field, related_field, None)
                        field.save()
                        setattr(external, related_field, form.instance)
                        external.save()
                    # Return success url
                    answer = HttpResponseRedirect(self.get_success_url())
                except CodenerixException:
                    answer = super(GenUpdateBridge, self).form_invalid(form)
            else:
                errors = form._errors.setdefault(field, ErrorList())
                errors.append(error_message[0])
                answer = super(GenUpdateBridge, self).form_invalid(form)

        # Return result
        return answer