From 985e6c224be0681d7a753b9ded4646024d332bc7 Mon Sep 17 00:00:00 2001
From: Matt Westcott <matt@west.co.tt>
Date: Thu, 14 Mar 2019 18:23:50 +0100
Subject: [PATCH] [2.2.x] Fixed #30254 -- Allowed model metaclasses to access
 the attribute dict in __init__().

Regression in a68ea231012434b522ce45c513d84add516afa60.

Backport of 58ad030d05fa50cfed327368ab61defca3303e02 from master.
---
 django/db/models/base.py     | 12 ++++++++----
 tests/model_regress/tests.py | 20 +++++++++++++++++++-
 2 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/django/db/models/base.py b/django/db/models/base.py
index faed79cc2f..2f961a4393 100644
--- a/django/db/models/base.py
+++ b/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
diff --git a/tests/model_regress/tests.py b/tests/model_regress/tests.py
index e3977ee316..28eed87008 100644
--- a/tests/model_regress/tests.py
+++ b/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):