#32433: Delete distinct produces an unexpected and potentially harmful SQL
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Reporter: egism | Owner: nobody
Type: | Status: new
Uncategorized |
Component: Database | Version: 3.1
layer (models, ORM) | Keywords: orm, delete,
Severity: Normal | distinct, postgresql
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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I was looking for a way to delete the first Comment of each Post (a sample
domain). Since I know that every new Post starts with a system generated
comment I decided to go with:
`Comment.objects.order_by('post_id',
'created_at').distinct('post_id').delete()`
Before proceeding I tested it with:
`Comment.objects.order_by('post_id',
'created_at').distinct('post_id').count()`
Made sure the result actually contains what I needed and proceeded with
the `delete()`. The result was rather surprising. I was notified that the
whole table was wiped clean. I then checked the actual SQL that was
executed and it was a simple `DELETE FROM comments;`.
As an ORM user, I would say it is the worst outcome possible and I would
at least expect an error in such a case or ideally a SQL of what I was
trying to achieve. At the same time, `count` and `delete` produces an
inconsistent result which is even more mistaking.
Potential solutions:
* raise an error with a decent explanation
* produce a desired SQL according to the query
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Ticket URL: <https://code.djangoproject.com/ticket/32433>
Django <https://code.djangoproject.com/>
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