Vicious cycle: Cannot migrate a DB that uses a widget using a model

[Victor Porton]

When I try to migrate my project from empty DB state, it falls into a vicious cycle: Trying to migrate it requests Country.objects.all() because it has form fields using PlaceWidget but this requires it to be already migrated.

How to solve this problem?

class PlaceWidget(widgets.MultiWidget):
    template_name = 'core/widgets/placewidget.html'

    class Media:
        js = ('js/placewidget.js',)

    def __init__(self, attrs=None):
        _widgets = (
            widgets.Select(choices=[('', '-')] + [(c.pk, c.name) for c in Country.objects.all().order_by('name')],
                           attrs={'onchange': "update_places_list(1)", **(attrs or {})}),
            widgets.Select(choices=[('', '-')],
                           attrs={'onchange': "update_places_list(2)", **(attrs or {})}),
            widgets.Select(choices=[('', '-')],
                           attrs={'onchange': "update_places_list(3)", **(attrs or {})}),
            widgets.Select(choices=[('', '-')],
                           attrs={'onchange': "update_places_list(4)", **(attrs or {})}),
            widgets.Select(choices=[('', '-')])
        )
        super().__init__(_widgets, attrs)

    def decompress(self, value):
        if value:
            return [value['country'].pk if value['country'] else None,
                    value['region'].pk if value['region'] else None,
                    value['subregion'].pk if value['subregion'] else None,
                    value['city'].pk if value['city'] else None,
                    value['district'].pk if value['district'] else None]
        return [None, None, None, None, None]

[Simon Charette]

Hello Victor,

You should avoid performing database queries at module loading level as the code will

necessary crash if the database is missing or unmigrated.

I suggest you lazily define widgets instead by using a cached property[0]

class PlaceWidget(widgets.MultiWidget):

    def __init__(self, attrs=None):

        # Bypass MultiWidget.__init__ to allow lazy definition of widgets.

        super(widgets.MultiWidget, self).__init__(attrs)

    @cached_property

    def widgets(self):

        return (

            ... # the _widgets you defined in your __init__.

        )

This should make sure the Country.objects.order_by('name') query is only executed

when .widgets is accessed for the first time on a PlaceWidget instance instead of at

initialization time.

Cheers,

Simon

[0] https://docs.djangoproject.com/en/2.2/topics/performance/#cached-property