Fixed #10919 -- Added delete_confirmation_max_display to ModelAdmin.

The new ModelAdmin.delete_confirmation_max_display attribute allows
limiting the number of related objects shown on the delete confirmation
page. When the limit is reached, a "…and N more objects." message is shown.

The feature relies on a new truncated_unordered_list template filter
added to django.contrib.admin.templatetags.admin_filters.

Thanks Jacob Tyler Walls for the review and guidance, Tobias McNulty for the report,
and terminator14 for the solution suggested.

1 files changed, 20 insertions, 0 deletions

diff --git a/django/contrib/admin/checks.py b/django/contrib/admin/checks.py
index d14515e8a4..7baaa38a29 100644
--- a/django/contrib/admin/checks.py
+++ b/django/contrib/admin/checks.py
@@ -842,6 +842,7 @@ class ModelAdminChecks(BaseModelAdminChecks):
             *self._check_search_fields(admin_obj),
             *self._check_date_hierarchy(admin_obj),
             *self._check_actions(admin_obj),
+            *self._check_delete_confirmation_max_display(admin_obj),
         ]
 
     def _check_save_as(self, obj):
@@ -1089,6 +1090,25 @@ class ModelAdminChecks(BaseModelAdminChecks):
         else:
             return []
 
+    def _check_delete_confirmation_max_display(self, obj):
+        """Check that delete_confirmation_max_display is
+        a non-negative integer or None."""
+
+        if obj.delete_confirmation_max_display is None:
+            return []
+        if (
+            not isinstance(obj.delete_confirmation_max_display, int)
+            or obj.delete_confirmation_max_display < 0
+        ):
+            return must_be(
+                "a non-negative integer or None",
+                option="delete_confirmation_max_display",
+                obj=obj,
+                id="admin.E131",
+            )
+        else:
+            return []
+
     def _check_list_per_page(self, obj):
         """Check that list_per_page is an integer."""