Django admin annotate the page's objects rather than the queryset

admin.py

# There will come a time when you want to show more information on the admin
# list view, but performing the annotation on the entire queryset kills
# performance. These classes will allow you to apply a QuerySet annotation
# to only those objects that are rendered on the current page.

# Constraints:
# - This adds one additional query to your request.
# - The annotated columns can't be used in ordering.
# - You need to define functions to access the annotated fields on the object.

from django.contrib import admin
from django.core.paginator import Paginator, Page
from django.db.models import Count, Exists, OuterRef

from myapp.models import MyModel


class AnnotatedPaginator(Paginator):
    """
    Apply a dictionary of annotations to the page for the paginator.
    """
    
    def __init__(self, *args, annotations=None, **kwargs):
        self.annotations = annotations
        super().__init__(*args, **kwargs)

    def _get_page(self, object_list, *args, **kwargs):
        """
        Return an instance of a single page.

        This will change the object_list into an actual list.
        It will make an additional query to the database to look up
        the values to be manually set on the object.
        """
        objects = list(object_list)
        if objects and self.annotations:
            # Make another query for this model type to gather the annotated fields.
            annotated_queryset = (
                objects[0]
                ._meta.model.objects.filter(id__in=[obj.id for obj in objects])
                .annotate(**self.annotations)
            )
            # Create a map to associate the original objects to the annotated values.
            annotated_maps = {
                annotated_map.pop("id"): annotated_map
                for annotated_map in annotated_queryset.values(
                    "id", *self.annotations.keys()
                )
            }
            # Associated the annotated values to the original objects.
            for obj in objects:
                for key, value in annotated_maps[obj.id].items():
                    setattr(obj, key, value)
        return Page(objects, *args, **kwargs)


class AdminAnnotatedPageMixin:
    """
    Extend the ModelAdmin functionality to utilize AnnotatedPaginator
    """
    
    paginator = AnnotatedPaginator
    page_annotations = None

    def get_paginator(
        self, request, queryset, per_page, orphans=0, allow_empty_first_page=True
    ):
        return self.paginator(
            queryset,
            per_page,
            orphans,
            allow_empty_first_page,
            annotations=self.page_annotations,
        )


@admin.site.register(MyModel)
class MyModelAdmin(AdminAnnotatedPageMixin, admin.ModelAdmin):
    page_annotations = {
        "related_field_count": Count("related_field"),
        "special_case_exists": Exists(
            OtherModel.objects.filter(relation_id=OuterRef("id"))[:1]
        ),
    }
    
    @admin.display(description="Related Count")
    def related_field_count(self, obj):
        return obj.related_field_count
    
    @admin.display(description="Is Special", boolean=True)
    def special_case_exists(self, obj):
        return obj.special_case_exists