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
34555	Dynamic model fields without using a metaclass	hottwaj	nobody	"I'd like to be able to write some abstract model ""templates"" that can be re-used/customised in various other django apps.  Some of these templates require ForeignKeys to other ""templates"" which are also abstract.  Obviously ForeignKeys to Abstract models are not possible

Having used metaclasses in a similar situation before, but hoping to find a less complex approach, I thought I might be able to implement this by providing a __init_subclass__ method on my ""template"" model, with that method designed to set up the ForeignKey to a given concrete model, but this does not work.  It seems that __init_subclass__ is called after ModelBase.__new__ collects fields that require ""contribute_to_class"", so the fields I add in __init_subclass__ are never ""seen"" by e.g. the migration builder.

Example approach below (but note this does not work for reasons explained above):

{{{#!python
from django.db.models import Model, ForeignKey, CASCADE, CharField

class BaseBookModel(Model):
    class Meta:
        abstract = True

    def __init_subclass__(cls, author_model_cls: Type[Model], **kwargs,):
        super().__init_subclass__(**kwargs)
        author = ForeignKey(author_model_cls, on_delete = CASCADE)
        cls.add_to_class('author', author)

class Author(Model):
    name = CharField(max_len = 256, unique = True)

class Book(BaseBookModel, author_model_cls = Author):
    pass
}}}

Essentially what I'd like is some way of doing some extra work after BaseModel.__new__ is called without resorting to having to write a metaclass for BaseBookModel - I'm just adding some extra fields and a metaclass is complex for most mortals to read let alone write.  There does not seem to be an appropriate hook method that I can override to do this for every subclass of BaseBookModel

Thanks for reading and appreciate any thoughts!
"	New feature	new	Database layer (models, ORM)	4.2	Normal		ModelBase init_subclass		Unreviewed	0	0	0	0	0	0