Add encrypted admin w/"no count" pagination

Existing approaches to "no count" pagination do not seem to include
support for pagination in the Django admin, so:

- Subclass Django's core Paginator
  - override count

- Subclass Django's contrib admin view ChangeList.get_results
  - Use len instead of count
  - Ruff fixes for try/except

- Subclass Django's contrib ModelAdmin
  - override get_paginator
  - override get_changelist

Author: aclark4life

django_mongodb_backend/admin.py

@@ -0,0 +1,56 @@
+from django.contrib import admin
+from django.contrib.admin.options import IncorrectLookupParameters
+from django.contrib.admin.views.main import ChangeList
+from django.core.paginator import InvalidPage, Paginator
+from django.utils.functional import cached_property
+
+
+class EncryptedPaginator(Paginator):
+    @cached_property
+    def count(self):
+        return len(self.object_list)
+
+
+class EncryptedChangeList(ChangeList):
+    def get_results(self, request):
+        """
+        This is django.contrib.admin.views.main.ChangeList.get_results with
+        a single modification to avoid COUNT queries.
+        """
+        paginator = self.model_admin.get_paginator(request, self.queryset, self.list_per_page)
+        result_count = paginator.count
+        if self.model_admin.show_full_result_count:
+            # Modification: avoid COUNT query by using len() on the root queryset
+            full_result_count = len(self.root_queryset)
+        else:
+            full_result_count = None
+        can_show_all = result_count <= self.list_max_show_all
+        multi_page = result_count > self.list_per_page
+        if (self.show_all and can_show_all) or not multi_page:
+            result_list = self.queryset._clone()
+        else:
+            try:
+                result_list = paginator.page(self.page_num).object_list
+            except InvalidPage as err:
+                raise IncorrectLookupParameters from err
+        self.result_count = result_count
+        self.show_full_result_count = self.model_admin.show_full_result_count
+        self.show_admin_actions = not self.show_full_result_count or bool(full_result_count)
+        self.full_result_count = full_result_count
+        self.result_list = result_list
+        self.can_show_all = can_show_all
+        self.multi_page = multi_page
+        self.paginator = paginator
+
+
+class EncryptedModelAdmin(admin.ModelAdmin):
+    """
+    A ModelAdmin that uses EncryptedPaginator and EncryptedChangeList
+    to avoid COUNT queries in the admin changelist.
+    """
+
+    def get_paginator(self, request, queryset, per_page):
+        return EncryptedPaginator(queryset, per_page)
+
+    def get_changelist(self, request, **kwargs):
+        return EncryptedChangeList

docs/ref/contrib/admin.rst

@@ -0,0 +1,34 @@
+=====
+Admin
+=====
+
+Django MongoDB Backend supports the Django admin interface. To enable it, ensure
+that you have :ref:`specified the default pk field
+<specifying the-default-pk-field>` for the
+:class:`~django.contrib.admin.apps.AdminConfig` class as described in the
+:doc:`Getting Started </intro/configure>` guide.
+
+``EncryptedModelAdmin``
+=======================
+
+.. class:: EncryptedModelAdmin
+
+    .. versionadded:: 5.2.3
+
+    A :class:`~django.contrib.admin.ModelAdmin` subclass that supports models
+    with encrypted fields. Use this class as a base class for your model's admin
+    class to ensure that encrypted fields are handled correctly in the admin
+    interface.
+
+    Register encrypted models with the Django admin using the
+    ``EncryptedModelAdmin`` as shown below::
+
+        # myapp/admin.py
+        from django.contrib import admin
+        from django_mongodb_backend.admin import EncryptedModelAdmin
+        from .models import Patient
+
+
+        @admin.register(Patient)
+        class PatientAdmin(EncryptedModelAdmin):
+            pass