allows Fields to be added to Models in __init_subclass__ without affecting existing ModelBase.__new__ API

django/db/models/base.py

@@ -118,6 +118,11 @@ class ModelBase(type):
             else:
                 new_attrs[obj_name] = obj
         new_class = super_new(cls, name, bases, new_attrs, **kwargs)
+        # update contributable_attrs with any such attributes added to new_class - these came from __init_subclass__
+        for obj_name, obj in dict(vars(new_class)).items():
+            if _has_contribute_to_class(obj):
+                contributable_attrs[obj_name] = obj
+                delattr(new_class, obj_name)
 
         abstract = getattr(attr_meta, "abstract", False)
         meta = attr_meta or getattr(new_class, "Meta", None)

tests/model_inheritance/tests.py

@@ -249,6 +249,24 @@ class ModelInheritanceTests(TestCase):
 
         self.assertEqual(saved_kwargs, kwargs)
 
+    @isolate_apps("model_inheritance")
+    def test_fields_can_be_added_in_init_subclass(self):
+        class BaseBookModel(models.Model):
+            class Meta:
+                abstract = True
+
+            def __init_subclass__(cls, author_model_cls, **kwargs):
+                super().__init_subclass__(**kwargs)
+                cls.author = models.ForeignKey(author_model_cls, on_delete = models.CASCADE)
+
+        class Author(models.Model):
+            name = models.CharField(max_length = 256, unique = True)
+
+        class Book(BaseBookModel, author_model_cls = Author):
+            pass
+
+        self.assertIsInstance(Book._meta.get_field('author'), models.ForeignKey)
+
     @isolate_apps("model_inheritance")
     def test_set_name(self):
         class ClassAttr: