admin.add_preserved_filters() drops repeated query parameters

[ollehu]

django.contrib.admin.templatetags.admin_urls.add_preserved_filters() loses repeated query parameters when restoring changelist filters from an admin change form.

Reproduction

Use a changelist URL containing repeated parameters:

/admin/?X__id__exact=57&X__id__exact=54

After opening an object, the change form receives:

_changelist_filters=X__id__exact%3D57%26X__id__exact%3D54

The admin Close/back URL is then generated as:

/admin/?X__exact=54

The first value has been discarded.

Cause

add_preserved_filters() repeatedly converts parse_qsl() results to a regular dictionary.

I have a fix prepared that we already use in a fork and can issue a PR when this gets accepted.

[Sarah Boyce]

Replicated thank you. Please check "has patch" when you have created your PR

tests/admin_views/tests.py

@@ -9095 +9095 @@
                     add_preserved_filters(context, url),
                 )
 
+    def test_multiple_changelist_filter_values(self):
+        changelist_url = reverse("admin:auth_user_changelist")
+        context = {
+            "opts": User._meta,
+            "preserved_filters": urlencode(
+                {
+                    "_changelist_filters": urlencode(
+                        [
+                            ("id__exact", 1),
+                            ("id__exact", 2),
+                        ]
+                    )
+                }
+            ),
+        }
+        self.assertEqual(
+            add_preserved_filters(context, changelist_url),
+            f"{changelist_url}?id__exact=1&id__exact=2",
+        )