Django's SELECT Before UPDATE Behavior with Related Fields

Based on the information available in the Django documentation, the behavior you're observing (a preliminary SELECT query before an UPDATE when filtering on related fields) isn't explicitly documented in the sections I have access to.

You've identified an important potential race condition issue: when Django splits the operation into two queries (SELECT followed by UPDATE) instead of performing a single UPDATE query, there's a window where another transaction could modify the data between these two operations.

Potential Workaround: Using select_for_update()

If you're concerned about race conditions, you might want to use Django's select_for_update() method, which locks the selected rows until your transaction is complete:

PYTHONCopied
from django.db import transaction

with transaction.atomic():
    choices = Choice.objects.filter(pk=1) \
                           .filter(question__question_text="TEST") \
                           .select_for_update()
    
    if choices.exists():
        choices.update(votes=100)

This approach will lock the rows during the SELECT operation, preventing other transactions from modifying them until your transaction completes.

MySQL-Specific Considerations

When using select_for_update() with MySQL, the documentation notes:

When using select_for_update() on MySQL, make sure you filter a queryset against at least a set of fields contained in unique constraints or only against fields covered by indexes. Otherwise, an exclusive write lock will be acquired over the full table for the duration of the transaction.

In your case, since you're filtering on pk=1, which is a primary key, this should be fine.

Additional Notes

• The behavior you're seeing might be related to how Django handles JOINs in UPDATE statements, especially with MySQL.
• This behavior could potentially change in future Django versions.
• If this is causing significant issues in your application, you might want to consider filing a documentation improvement request or even a bug report with the Django project.