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@jurrian
Last active Jan 11, 2023
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Patched ForeignKeyRawIdWidget for prefetching
from django.contrib.admin import widgets
from django.contrib.admin.options import get_ul_class
from django.contrib.admin.widgets import AutocompleteSelect
from django.forms import boundfield, models
from django.urls import reverse
from django.urls.exceptions import NoReverseMatch
from django.utils.text import Truncator
from django.utils.translation import gettext_lazy as _
class ForeignKeyRawIdWidget(widgets.ForeignKeyRawIdWidget):
def format_value(self, value):
"""Try to return the `pk` if value is an object, otherwise just return
the value as fallback."""
if value == '' or value is None:
return None
try:
return str(value.pk)
except AttributeError:
return str(value)
def label_and_url_for_value(self, value):
"""Instead of the original we do not have do a `get()` anymore instead
access the instance directly so when value is prefetched this will
prevent additional queries."""
try:
pk = value.pk
meta = value._meta
except AttributeError:
# Fallback for compatibility with plain pk values
return super().label_and_url_for_value(value)
try:
url = reverse(
f'{self.admin_site.name}:{meta.app_label}_{meta.object_name.lower()}_change',
args=(pk,)
)
except NoReverseMatch:
url = '' # Admin not registered for target model.
return Truncator(value).words(14), url
class BoundField(boundfield.BoundField):
def value(self):
"""Return the instance instead of plain value if possible.
In order for `ForeignKeyRawIdWidget` to access the model instance directly
we grab if from the form if available."""
if type(self.field.widget) == ForeignKeyRawIdWidget:
try:
return getattr(self.form.instance, self.name)
except AttributeError:
pass
# Otherwise default behaviour
return super().value()
class ModelChoiceField(models.ModelChoiceField):
def get_bound_field(self, form, field_name):
"""Return our custom `BoundField`."""
return BoundField(form, self, field_name)
class RawIdWidgetAdminMixin:
def formfield_for_foreignkey(self, db_field, request, **kwargs):
"""ModelAdmin mixin that uses a custom `ForeignKeyRawIdWidget`.
This prevents extra queries when the queryset has been prefetched using
`prefetch_related()`. Only works when `raw_id_fields` is filled."""
if db_field.name not in self.raw_id_fields:
# If we are not using raw_id_fields then skip the whole thing
return super().formfield_for_foreignkey(db_field, request, **kwargs)
db = kwargs.get('using')
if 'widget' not in kwargs:
if db_field.name in self.get_autocomplete_fields(request):
kwargs['widget'] = AutocompleteSelect(db_field.remote_field, self.admin_site, using=db)
elif db_field.name in self.raw_id_fields:
# Using our modified ForeignKeyRawIdWidget here instead
kwargs['widget'] = ForeignKeyRawIdWidget(db_field.remote_field, self.admin_site, using=db)
elif db_field.name in self.radio_fields:
kwargs['widget'] = widgets.AdminRadioSelect(attrs={
'class': get_ul_class(self.radio_fields[db_field.name]),
})
kwargs['empty_label'] = _('None') if db_field.blank else None
if 'queryset' not in kwargs:
queryset = self.get_field_queryset(db, db_field, request)
if queryset is not None:
kwargs['queryset'] = queryset
if isinstance(db_field.remote_field.model, str):
raise ValueError(
f"Cannot create form field for {db_field.name!r} yet, because "
"its related model {db_field.remote_field.model!r} has not been loaded yet"
)
return super(type(db_field), db_field).formfield(**{
# Using our modified ModelChoiceField here instead
'form_class': ModelChoiceField,
'queryset': db_field.remote_field.model._default_manager.using(db),
'to_field_name': db_field.remote_field.field_name,
**kwargs,
'blank': db_field.blank,
})
@jurrian
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jurrian commented Jan 11, 2023

I wrote an article about why this patch is necessary and how to use the mixin:
https://deepintodjango.com/reducing-queries-for-foreignkeys-in-django-admin-inlines

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