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
27408	Make QuerySet.bulk_create() populate fields on related models	Jarek Glowacki	nobody	"As of dj110 `bulk_create` returns pks for the objects it has created. Great! I want more.

Going to hop right into the use case:
{{{
class A(models.Model):
    pass


class B(models.Model):
    a = models.ForeignKey(A, on_delete=models.PROTECT)
}}}

{{{
a = A()
b = B(a=a)  # doesn't set `a_id`, since `a` has no id assigned yet.
A.objects.bulk_create([a])  # sets `a`'s id.
B.objects.bulk_create([b])  # error!
}}}
The first `bulk_create` call sets the id on `a`, but `b.a_id` remains `None`.
When running the second `bulk_create`, even though `b` has an `a` set, it fails to save `a` onto the `b` instance, due to `a_id` being None.

So if you imagine the pregeneration of several related models in a big loop, followed by several consecutive `bulk_create` calls we run into trouble. It's nicer than having to manually feed in ids during the loop as we needed to in the past, but it still needs some magic.

My current solution is running a little function between each `bulk_create` call, to fill the foreignkey ids in:
{{{
def fill_foreignkey_ids(objects, fields):
    for f in fields:
        for o in objects:
            setattr(o, '%s_id' % f, getattr(o, f).id)
}}}
Which makes:
{{{
a = A()
b = B(a=a)  # doesn't set `a_id`, since `a` has no id assigned yet.
bulk_a = [a]
bulk_b = [b]
A.objects.bulk_create(bulk_a)  # sets `a`'s id.
fill_foreignkey_ids(bulk_b, ['a'])
B.objects.bulk_create(bulk_b)  # now it works. Lovely.
}}}

But this is quite ugly.

This is more of a brainstorm ticket, as I'm not sure how to solve this nicely. Expecting every unsaved model instance to update its foreignkey ids when a `bulk_create` inserts them sounds seems silly. But maybe we could have the `bulk_create` method check whether `b.a.id` is set when `b.a_id` isn't?

I can submit a PR if this sounds like a reasonable thing to do. It feels a bit magic, but it would greatly improve the usefulness of this new feature."	New feature	closed	Database layer (models, ORM)	dev	Normal	wontfix	bulk_create foreign key id		Someday/Maybe	0	0	0	0	0	0