join on plain integerfield

[Marek Rouchal]

I need to access an existing database whose schema I cannot alter; I used the Django tool to extract the tables to create models.py, which worked great, and I already hacked some model definitions, e.g. to replace a simple IntegerField with a ForeignKey. But there are specific tables, where there is an IntegerField which may contain a ForeignKey from more than one table (think of a group and a user table; whether the reference is a user or a group, is defined by another column):

class Permission(models.Model):

    ...

    user_or_group_id = models.IntegerField(db_column='user_or_group_id', blank=True, null=True)

    is_group = models.IntegerField(db_column='is_group', blank=True, null=True)

So what I would need here is something like a union on ForeignKey (coming from either a User or a Group table) in the model, but I guess that does not exist?
So I used raw SQL for the required joins; I have not been able to formulate a Django queryset to join on a field which is not modeled with ForeignKey.
That is why I was asking for some better support of raw SQL, reference is this ticket: https://code.djangoproject.com/ticket/34168

Any suggestion how I can model such an integerfield, so that I can do joins on it using the Django ORM queryset syntax?

[Ryan Nowakowski]

That's sort of how generic relations[1] work. You might be able to make it fit.

[1] https://docs.djangoproject.com/en/4.1/ref/contrib/contenttypes/#generic-relations

[Marek Rouchal]

Thanks Ryan, that looks promising. I have to dig deeper, but upon first read I saw that an additional field would be required in the model definition; question: would that also require an additional column in the database (which I could not afford), or is it purely a „meta“ kind of field for ORM purposes only?

Ryan Nowakowski <ry...@fattuba.com> schrieb am Mo. 21. Nov. 2022 um 20:41:

That's sort of how generic relations[1] work. You might be able to make it fit.

[1] https://docs.djangoproject.com/en/4.1/ref/contrib/contenttypes/#generic-relations

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[Ryan Nowakowski]

It only uses 2 columns, the "type" of the relation and the id of the
related object. That's why I thought it might be a good fit for your
use case. If you can't use it as-is because your "type" column is a
boolean instead of a foreignkey to ContentType, you can look at the
GenericForeignKey code[1] to see if you can copy and adapt it.

[1] https://github.com/django/django/blob/main/django/contrib/contenttypes/fields.py#L25

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[Clive Bruton]

Try Django Relativity:

https://pypi.org/project/django-relativity/


-- Clive