Fixed #7475: fixed a possible race condition in ModelChoiceIterator. Thanks, esaj.

    def __iter__(self):

        if self.field.empty_label is not None:

            yield (u"", self.field.empty_label)

        for obj in self.queryset:

        if self.field.cache_choices:

            if self.field.choice_cache is None:

                self.field.choice_cache = [

                    (obj.pk, self.field.label_from_instance(obj))

                    for obj in self.queryset.all()

                ]

            for choice in self.field.choice_cache:

                yield choice

        else:

            for obj in self.queryset.all():

                yield (obj.pk, self.field.label_from_instance(obj))

        # Clear the QuerySet cache if required.

        if not self.field.cache_choices:

            self.queryset._result_cache = None

class ModelChoiceField(ChoiceField):

    """A ChoiceField whose choices are a model QuerySet."""

        Field.__init__(self, required, widget, label, initial, help_text,

                       *args, **kwargs)

        self.queryset = queryset

        self.choice_cache = None

    def _get_queryset(self):

        return self._queryset