Fixed #24146 -- Fixed a missing fields regression in admin checks.

This allows using get_field() early in the app loading process.

Thanks to PirosB3 and Tim Graham.

django/db/models/options.py

@@ -487,6 +487,12 @@ def _forward_fields_map(self):
 
     @cached_property
     def fields_map(self):
+        return self._get_fields_map()
+
+    def _get_fields_map(self):
+        # Helper method to provide a way to access this without caching it.
+        # For example, admin checks run before the app cache is ready and we
+        # need to be able to lookup fields before we cache the final result.
         res = {}
         fields = self._get_fields(forward=False, include_hidden=True)
         for field in fields:
@@ -531,20 +537,26 @@ def get_field(self, field_name, many_to_many=None):
 
             return field
         except KeyError:
-            # If the app registry is not ready, reverse fields are
-            # unavailable, therefore we throw a FieldDoesNotExist exception.
-            if not self.apps.ready:
+            pass
+
+        if m2m_in_kwargs:
+            # Previous API does not allow searching reverse fields.
+            raise FieldDoesNotExist('%s has no field named %r' % (self.object_name, field_name))
+
+        # If the app registry is not ready, reverse fields are probably
+        # unavailable, but try anyway.
+        if not self.apps.ready:
+            try:
+                # Don't cache results
+                return self._get_fields_map()[field_name]
+            except KeyError:
                 raise FieldDoesNotExist(
                     "%s has no field named %r. The app cache isn't ready yet, "
-                    "so if this is an auto-created related field, it won't "
+                    "so if this is an auto-created related field, it might not "
                     "be available yet." % (self.object_name, field_name)
                 )
 
         try:
-            if m2m_in_kwargs:
-                # Previous API does not allow searching reverse fields.
-                raise FieldDoesNotExist('%s has no field named %r' % (self.object_name, field_name))
-
             # Retrieve field instance by name from cached or just-computed
             # field map.
             return self.fields_map[field_name]

tests/model_meta/tests.py

@@ -169,24 +169,30 @@ def test_get_generic_relation(self):
         self.assertEqual(field_info[1:], (None, True, False))
         self.assertIsInstance(field_info[0], GenericRelation)
 
-    def test_get_fields_only_searches_forward_on_apps_not_ready(self):
+    def test_get_fields_when_apps_not_ready(self):
         opts = Person._meta
         # If apps registry is not ready, get_field() searches over only
         # forward fields.
         opts.apps.ready = False
+        # Clear cached data.
+        opts.__dict__.pop('fields_map', None)
         try:
             # 'data_abstract' is a forward field, and therefore will be found
             self.assertTrue(opts.get_field('data_abstract'))
             msg = (
-                "Person has no field named 'relating_baseperson'. The app "
+                "Person has no field named 'some_missing_field'. The app "
                 "cache isn't ready yet, so if this is an auto-created related "
-                "field, it won't be available yet."
+                "field, it might not be available yet."
             )
-            # 'data_abstract' is a reverse field, and will raise an exception
             with self.assertRaisesMessage(FieldDoesNotExist, msg):
-                opts.get_field('relating_baseperson')
+                opts.get_field('some_missing_field')
+            # Be sure it's not cached
+            self.assertNotIn('fields_map', opts.__dict__)
         finally:
             opts.apps.ready = True
+        # At this point searching a related field would cache fields_map
+        opts.get_field('relating_baseperson')
+        self.assertIn('fields_map', opts.__dict__)
 
 
 class RelationTreeTests(TestCase):