Fixed #18729 -- Made admin changelist not use distinct() if a list_filter doesn't require it.

django/contrib/admin/views/main.py

@@ -125,11 +125,16 @@ class ChangeList(object):
                     if not isinstance(field, models.Field):
                         field_path = field
                         field = get_fields_from_path(self.model, field_path)[-1]
+
+                    lookup_params_count = len(lookup_params)
                     spec = field_list_filter_class(
                         field, request, lookup_params,
                         self.model, self.model_admin, field_path=field_path
                     )
-                    # Check if we need to use distinct()
+                    # field_list_filter_class removes any lookup_params it
+                    # processes. If that happened, check if distinct() is
+                    # needed to remove duplicate results.
+                    if lookup_params_count > len(lookup_params):
                         use_distinct = use_distinct or lookup_needs_distinct(self.lookup_opts, field_path)
                 if spec and spec.has_output():
                     filter_specs.append(spec)

tests/admin_changelist/tests.py

@@ -445,6 +445,22 @@ class ChangeListTests(TestCase):
         # There's only one Concert instance
         self.assertEqual(cl.queryset.count(), 1)
 
+    def test_no_distinct_for_m2m_in_list_filter_without_params(self):
+        """
+        If a ManyToManyField is in list_filter but isn't in any lookup params,
+        the changelist's query shouldn't have distinct.
+        """
+        m = BandAdmin(Band, custom_site)
+        for lookup_params in ({}, {'name': 'test'}):
+            request = self.factory.get('/band/', lookup_params)
+            cl = ChangeList(request, Band, *get_changelist_args(m))
+            self.assertFalse(cl.queryset.query.distinct)
+
+        # A ManyToManyField in params does have distinct applied.
+        request = self.factory.get('/band/', {'genres': '0'})
+        cl = ChangeList(request, Band, *get_changelist_args(m))
+        self.assertTrue(cl.queryset.query.distinct)
+
     def test_pagination(self):
         """
         Regression tests for #12893: Pagination in admins changelist doesn't