[1.9.x] Fixed #25685 -- Fixed a duplicate query regression on deletion of proxied models.

Thanks to Trac alias ppetrid for the report and Tim for the review.

Conflicts:
	tests/delete/tests.py

Forward port of 7c3ef19978b36b61db88a519f799f1ce8d019679 from stable/1.8.x

4 files changed, 22 insertions, 2 deletions

diff --git a/django/db/models/deletion.py b/django/db/models/deletion.py
index a8c98f5b83..e067559ccb 100644
--- a/django/db/models/deletion.py
+++ b/django/db/models/deletion.py
@@ -58,9 +58,9 @@ def get_candidate_relations_to_delete(opts):
     candidate_models = {opts}
     candidate_models = candidate_models.union(opts.concrete_model._meta.proxied_children)
     # For each model, get all candidate fields.
-    candidate_model_fields = chain.from_iterable(
+    candidate_model_fields = set(chain.from_iterable(
         opts.get_fields(include_hidden=True) for opts in candidate_models
-    )
+    ))
     # The candidate relations are the ones that come from N-1 and 1-1 relations.
     # N-N  (i.e., many-to-many) relations aren't candidates for deletion.
     return (
diff --git a/docs/releases/1.8.7.txt b/docs/releases/1.8.7.txt
index 8ffae59b21..df0ff41159 100644
--- a/docs/releases/1.8.7.txt
+++ b/docs/releases/1.8.7.txt
@@ -39,3 +39,6 @@ Bugfixes
 
 * Fixed ``Model.refresh_from_db()`` updating of ``ForeignKey`` fields with
   ``on_delete=models.SET_NULL`` (:ticket:`25715`).
+
+* Fixed a duplicate query regression in 1.8 on proxied model deletion
+  (:ticket:`25685`).
diff --git a/tests/delete/models.py b/tests/delete/models.py
index a7838c4510..60e7f5957c 100644
--- a/tests/delete/models.py
+++ b/tests/delete/models.py
@@ -99,6 +99,12 @@ class Avatar(models.Model):
     desc = models.TextField(null=True)
 
 
+# This model is used to test a duplicate query regression (#25685)
+class AvatarProxy(Avatar):
+    class Meta:
+        proxy = True
+
+
 class User(models.Model):
     avatar = models.ForeignKey(Avatar, models.CASCADE, null=True)
 
diff --git a/tests/delete/tests.py b/tests/delete/tests.py
index 60c61ed281..c50f95e126 100644
--- a/tests/delete/tests.py
+++ b/tests/delete/tests.py
@@ -413,6 +413,17 @@ class DeletionTests(TestCase):
         for k, v in existed_objs.items():
             self.assertEqual(deleted_objs[k], v)
 
+    def test_proxied_model_duplicate_queries(self):
+        """
+        #25685 - Deleting instances of a model with existing proxy
+        classes should not issue multiple queries during cascade
+        deletion of referring models.
+        """
+        avatar = Avatar.objects.create()
+        # One query for the Avatar table and a second for the User one.
+        with self.assertNumQueries(2):
+            avatar.delete()
+
 
 class FastDeleteTests(TestCase):