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
Le mercredi 19 juin 2019 09:40:23 UTC-4, Victor Porton a écrit :
>
> 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]
>
>
>
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