#36915: Add support for select_for_update for implicit transactions
-------------------------------------+-------------------------------------
Reporter: David | Type:
| Cleanup/optimization
Status: new | Component: Database
| layer (models, ORM)
Version: 6.0 | Severity: Normal
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Currently the [https://docs.djangoproject.com/en/6.0/ref/models/querysets
/#select-for-update select_for_update] method is only allowed inside
explicit transactions which have been started using `transaction.atomic`.
However any DML SQL statements can create an "implicit transaction" in
which it may be useful to get control on row-level locks.
What I would like to achieve is a way to perform the following statement
(inspired by [https://stackoverflow.com/a/44669989 this SO answer]):
{{{#!sql
UPDATE my_table
SET my_col = 1
WHERE id IN (
SELECT id
FROM my_table
WHERE ...
FOR UPDATE
);
}}}
As of now it is not possible to perform this kind of query:
{{{#!python
MyModel.objects.filter(pk__in=MyModel.objects.filter(...).select_for_update()).update(my_col=...)
# raises TransactionManagementError: select_for_update cannot be used
outside of a transaction.
}}}
The same can be accomplished by forcing the transaction
{{{#!python
with transaction.atomic():
MyModel.objects.filter(pk__in=MyModel.objects.filter(...).select_for_update()).update(my_col=...)
# works!
}}}
It would be useful to improve locks management in bulk-operations.
If it is too difficult to add this kind of support with the ORM it could,
at least, be documented as a tip in the docs.
--
Ticket URL: <https://code.djangoproject.com/ticket/36915>
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