id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
30254	Custom model metaclasses cannot access the attribute dict in __init__	Matt Westcott	nobody	"In Django <=2.2, it is possible for models to define a custom metaclass (as a subclass of `models.base.ModelBase`) and access the attribute dict of the class being defined:

{{{
from django.db import models


class PageBase(models.base.ModelBase):
    def __init__(cls, name, bases, dct):
        super(PageBase, cls).__init__(name, bases, dct)
        if 'magic' in dct:
            print(""enabling magic on %s"" % (name))


class Page(models.Model, metaclass=PageBase):
    magic = True

    title = models.CharField(max_length=255)
}}}

As of commit a68ea231012434b522ce45c513d84add516afa60, this fails because all attributes without a `contribute_to_class` method are popped from the dict in `ModelBase.__new__` .

(This pattern is used by Wagtail's Page model https://github.com/wagtail/wagtail/blob/3e1e67021e0a20783ed59e17b43e3c481897fce3/wagtail/core/models.py#L190 , so this is causing various failures against django stable/2.2.x.)"	Bug	closed	Database layer (models, ORM)	2.2	Release blocker	fixed		Sergey Fedoseev	Ready for checkin	1	0	0	0	0	0