[2.2.x] Fixed #30254 -- Allowed model metaclasses to access the attribute dict in __init__().

Regression in a68ea23101.

Backport of 58ad030d05 from master.

django/db/models/base.py

@@ -84,9 +84,12 @@ class ModelBase(type):
         # Pass all attrs without a (Django-specific) contribute_to_class()
         # method to type.__new__() so that they're properly initialized
         # (i.e. __set_name__()).
+        contributable_attrs = {}
         for obj_name, obj in list(attrs.items()):
-            if not _has_contribute_to_class(obj):
-                new_attrs[obj_name] = attrs.pop(obj_name)
+            if _has_contribute_to_class(obj):
+                contributable_attrs[obj_name] = obj
+            else:
+                new_attrs[obj_name] = obj
         new_class = super_new(cls, name, bases, new_attrs, **kwargs)
 
         abstract = getattr(attr_meta, 'abstract', False)
@@ -146,8 +149,9 @@ class ModelBase(type):
         if is_proxy and base_meta and base_meta.swapped:
             raise TypeError("%s cannot proxy the swapped model '%s'." % (name, base_meta.swapped))
 
-        # Add all attributes to the class.
-        for obj_name, obj in attrs.items():
+        # Add remaining attributes (those with a contribute_to_class() method)
+        # to the class.
+        for obj_name, obj in contributable_attrs.items():
             new_class.add_to_class(obj_name, obj)
 
         # All the fields of any type declared on this model

tests/model_regress/tests.py

@@ -2,9 +2,10 @@ import datetime
 from operator import attrgetter
 
 from django.core.exceptions import ValidationError
-from django.db import router
+from django.db import models, router
 from django.db.models.sql import InsertQuery
 from django.test import TestCase, skipUnlessDBFeature
+from django.test.utils import isolate_apps
 from django.utils.timezone import get_fixed_timezone
 
 from .models import (
@@ -217,6 +218,23 @@ class ModelTests(TestCase):
         m3 = Model3.objects.get(model2=1000)
         m3.model2
 
+    @isolate_apps('model_regress')
+    def test_metaclass_can_access_attribute_dict(self):
+        """
+        Model metaclasses have access to the class attribute dict in
+        __init__() (#30254).
+        """
+        class HorseBase(models.base.ModelBase):
+            def __init__(cls, name, bases, attrs):
+                super(HorseBase, cls).__init__(name, bases, attrs)
+                cls.horns = (1 if 'magic' in attrs else 0)
+
+        class Horse(models.Model, metaclass=HorseBase):
+            name = models.CharField(max_length=255)
+            magic = True
+
+        self.assertEqual(Horse.horns, 1)
+
 
 class ModelValidationTest(TestCase):
     def test_pk_validation(self):