Opened 19 months ago

Last modified 17 months ago

#30129 new New feature

Allow creating models with fields values as a Subquery() with F() expressions

Reported by: Charlie McBride Owned by:
Component: Database layer (models, ORM) Version: 2.1
Severity: Normal Keywords: Subquery, F, Query Expressions
Cc: Triage Stage: Accepted
Has patch: no Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no

Description (last modified by Charlie McBride)

I understand why F() expressions are generally disallowed inside insert statements, the columns you are referencing don't yet exist, so it wouldn't make any sense to do so. However, if you are performing an insert with a Subquery (as in the example below), it's possible to have otherwise valid statements rejected because of the blanket blacklist of F() expressions during inserts.

For example:

given the following models:

class Item(models.Model):
    pass

class ItemVersion(models.Model):
    item = models.ForeignKey(Item, related_name='versions')
    version_number = models.IntegerField(default=0)

    class Meta:
        unique_together = ('item', 'version_number',)

I would like to be able to do the following operation to mitigate race conditions as described here (https://docs.djangoproject.com/en/2.1/ref/models/expressions/#avoiding-race-conditions-using-f)

    item = Item.objects.create()
    # arbitrary number of other items created/destroyed etc
    item_version_2 = ItemVersion.objects.create(
        item=item,
        version_number=Subquery(
            item.versions.order_by('-version_number').annotate(
                max_version_number=Coalesce(Max('version_number'), 0)
            ).annotate(
                new_version_number=F('max_version_number) + 1
            ).values('new_version_number')
         )
     )

As written, I would expect the F() in the inner expression to always be resolvable, because it is in a Subquery (and not the result of an insert). However this query is blocked by the compiler because "F() expressions can only be used to update, not to insert." Would it be possible to allow F expressions in a Subquery even if it is being used in an insert? Is there an edge case that I'm missing that caused the team to not consider this?

Change History (5)

comment:1 Changed 19 months ago by Tim Graham

Easy pickings: unset
Summary: Request to Allow F() Expressions in Subquery() on InsertAllow creating models with fields values as a Subquery() with F() expressions

comment:2 Changed 19 months ago by Charlie McBride

Description: modified (diff)

comment:3 Changed 19 months ago by Tim Graham

Triage Stage: UnreviewedAccepted

It seems okay, if it's feasible to implement.

comment:4 Changed 17 months ago by birthdaysgift

Owner: changed from nobody to birthdaysgift
Status: newassigned

comment:5 Changed 17 months ago by birthdaysgift

Owner: birthdaysgift deleted
Status: assignednew
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