Refs #27852 -- Fixed object deletion to show all protected related objects rather than just the first one.

Thanks Anton Samarchyan for the initial patch.

django/db/models/deletion.py

@@ -265,6 +265,7 @@ class Collector:
         if keep_parents:
             parents = set(model._meta.get_parent_list())
         model_fast_deletes = defaultdict(list)
+        protected_objects = defaultdict(list)
         for related in get_candidate_relations_to_delete(model._meta):
             # Preserve parent reverse relationships if keep_parents=True.
             if keep_parents and related.model in parents:
@@ -292,7 +293,23 @@ class Collector:
                     ))
                     sub_objs = sub_objs.only(*tuple(referenced_fields))
                 if sub_objs:
-                    field.remote_field.on_delete(self, field, sub_objs, self.using)
+                    try:
+                        field.remote_field.on_delete(self, field, sub_objs, self.using)
+                    except ProtectedError as error:
+                        key = "'%s.%s'" % (
+                            error.protected_objects[0].__class__.__name__,
+                            field.name,
+                        )
+                        protected_objects[key] += error.protected_objects
+        if protected_objects:
+            raise ProtectedError(
+                'Cannot delete some instances of model %r because they are '
+                'referenced through protected foreign keys: %s.' % (
+                    model.__name__,
+                    ', '.join(protected_objects),
+                ),
+                chain.from_iterable(protected_objects.values()),
+            )
         for related_model, related_fields in model_fast_deletes.items():
             batches = self.get_del_batches(new_objs, related_fields)
             for batch in batches:

tests/delete/models.py

@@ -68,6 +68,10 @@ class A(models.Model):
     o2o_setnull = models.ForeignKey(R, models.SET_NULL, null=True, related_name="o2o_nullable_set")
 
 
+class B(models.Model):
+    protect = models.ForeignKey(R, models.PROTECT)
+
+
 def create_a(name):
     a = A(name=name)
     for name in ('auto', 'auto_nullable', 'setvalue', 'setnull', 'setdefault',

tests/delete/tests.py

@@ -1,12 +1,14 @@
 from math import ceil
 
 from django.db import IntegrityError, connection, models
-from django.db.models.deletion import Collector, RestrictedError
+from django.db.models.deletion import (
+    Collector, ProtectedError, RestrictedError,
+)
 from django.db.models.sql.constants import GET_ITERATOR_CHUNK_SIZE
 from django.test import TestCase, skipIfDBFeature, skipUnlessDBFeature
 
 from .models import (
-    B1, B2, MR, A, Avatar, Base, Child, DeleteBottom, DeleteTop, GenericB1,
+    B1, B2, MR, A, Avatar, B, Base, Child, DeleteBottom, DeleteTop, GenericB1,
     GenericB2, GenericDeleteBottom, HiddenUser, HiddenUserProfile, M, M2MFrom,
     M2MTo, MRNull, Origin, P, Parent, R, RChild, RChildChild, Referrer, S, T,
     User, create_a, get_default_r,
@@ -72,11 +74,22 @@ class OnDeleteTests(TestCase):
         a = create_a('protect')
         msg = (
             "Cannot delete some instances of model 'R' because they are "
-            "referenced through a protected foreign key: 'A.protect'"
+            "referenced through protected foreign keys: 'A.protect'."
         )
         with self.assertRaisesMessage(IntegrityError, msg):
             a.protect.delete()
 
+    def test_protect_multiple(self):
+        a = create_a('protect')
+        B.objects.create(protect=a.protect)
+        msg = (
+            "Cannot delete some instances of model 'R' because they are "
+            "referenced through protected foreign keys: 'A.protect', "
+            "'B.protect'."
+        )
+        with self.assertRaisesMessage(ProtectedError, msg):
+            a.protect.delete()
+
     def test_do_nothing(self):
         # Testing DO_NOTHING is a bit harder: It would raise IntegrityError for a normal model,
         # so we connect to pre_delete and set the fk to a known value.