forms.py 文件源码

def render_options(self, *args):
        """Render only selected options and set QuerySet from :class:`ModelChoicesIterator`."""
        try:
            selected_choices, = args
        except ValueError:
            choices, selected_choices = args
            choices = chain(self.choices, choices)
        else:
            choices = self.choices
        selected_choices = {force_text(v) for v in selected_choices}
        output = ['<option></option>' if not self.is_required and not self.allow_multiple_selected else '']
        if isinstance(self.choices, ModelChoiceIterator):
            # Mejora significativa de rendimiernto con esta condición
            if self.queryset is None:
                self.queryset = self.choices.queryset
            selected_choices = {c for c in selected_choices
                                if c not in self.choices.field.empty_values}
            choices = {(obj.pk, self.label_from_instance(obj))
                       for obj in self.choices.queryset.filter(pk__in=selected_choices)}
        else:
            choices = {(k, v) for k, v in choices if force_text(k) in selected_choices}
        for option_value, option_label in choices:
            output.append(self.render_option(selected_choices, option_value, option_label))
        return '\n'.join(output)