Patched ForeignKeyRawIdWidget for prefetching

raw_id_widget.py

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,
        })

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