Fixed #21129 -- Prevented admin filter params modifications from throwing an exception.

Thanks Tuttle for the report.

django/contrib/admin/options.py

@@ -348,6 +348,10 @@ class BaseModelAdmin(six.with_metaclass(RenameBaseModelAdminMethods)):
                 # later.
                 return True
             if hasattr(field, 'rel'):
+                if field.rel is None:
+                    # This property or relation doesn't exist, but it's allowed
+                    # since it's ignored in ChangeList.get_filters().
+                    return True
                 model = field.rel.to
                 rel_name = field.rel.get_related_field().name
             elif isinstance(field, RelatedObject):

tests/modeladmin/tests.py

@@ -95,6 +95,18 @@ class ModelAdminTests(TestCase):
         form = ma.get_formset(None).form
         self.assertEqual(form._meta.fields, ['day', 'transport'])
 
+    def test_lookup_allowed_allows_nonexistent_lookup(self):
+        """
+        Ensure that a lookup_allowed allows a parameter
+        whose field lookup doesn't exist.
+        Refs #21129.
+        """
+        class BandAdmin(ModelAdmin):
+            fields = ['name']
+
+        ma = BandAdmin(Band, self.site)
+        self.assertTrue(ma.lookup_allowed('name__nonexistent', 'test_value'))
+
     def test_field_arguments(self):
         # If we specify the fields argument, fieldsets_add and fielsets_change should
         # just stick the fields into a formsets structure and return it.