#27408: Multiple consecutive `bulk_create`s with FK.
-------------------------------------+-------------------------------------
Reporter: Jarek | Owner: nobody
Glowacki |
Type: New | Status: new
feature |
Component: Database | Version: master
layer (models, ORM) | Keywords: bulk_create foreign
Severity: Normal | key id
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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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.bulk_create([a]) # sets `a`'s id.
B.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.bulk_create(bulk_a) # sets `a`'s id.
fill_foreignkey_ids(bulk_b, ['a'])
B.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.
--
Ticket URL: <https://code.djangoproject.com/ticket/27408>
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