Opened 6 years ago
Closed 22 months ago
#30129 closed New feature (fixed)
Allow creating models with fields values as a Subquery() with F() expressions
Reported by: | Charlie McBride | Owned by: | Sarah Boyce |
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Component: | Database layer (models, ORM) | Version: | 2.1 |
Severity: | Normal | Keywords: | Subquery, F, Query Expressions |
Cc: | Triage Stage: | Accepted | |
Has patch: | yes | Needs documentation: | no |
Needs tests: | no | Patch needs improvement: | no |
Easy pickings: | no | UI/UX: | no |
Description (last modified by )
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 (9)
comment:1 by , 6 years ago
Easy pickings: | unset |
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Summary: | Request to Allow F() Expressions in Subquery() on Insert → Allow creating models with fields values as a Subquery() with F() expressions |
comment:2 by , 6 years ago
Description: | modified (diff) |
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comment:3 by , 6 years ago
Triage Stage: | Unreviewed → Accepted |
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comment:6 by , 22 months ago
I think this is resolved (I'm not sure when), raised a PR just to show that I think it's been fixed https://github.com/django/django/pull/16451 (but I will close the PR later).
I feel like I'm not supposed to close tickets so I'll let a DSF team member check I haven't missed anything and close the ticket.
comment:7 by , 22 months ago
Has patch: | set |
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Owner: | set to |
Status: | new → assigned |
PR to add a test for this.
comment:8 by , 22 months ago
I did a bisect and confirmed this was fixed in Django 3.0 by 35431298226165986ad07e91f9d3aca721ff38ec.
comment:11 by , 22 months ago
Resolution: | → fixed |
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Status: | assigned → closed |
It seems okay, if it's feasible to implement.