Django UniqueConstraint and IntegrityError Behavior

Yes, the behavior you're experiencing is related to the order of operations when saving and deleting objects in Django with unique constraints.

What's Happening

When you're trying to update an object and delete another object in the same transaction, Django's default behavior is to execute the SQL statements in a specific order:

1. First, it attempts to update the existing record with the new value
2. Then, it would delete the conflicting record

This causes the IntegrityError because at the moment of the update, the database sees two records with the same value for the unique field - the one you're updating and the one you're planning to delete (but haven't deleted yet).

Solutions

You have a few options to handle this situation:

1. Use a transaction with the correct order of operations

PYTHONCopied
from django.db import transaction

with transaction.atomic():
    # First delete the conflicting object
    conflicting_object.delete()
    
    # Then update your object
    my_object.my_field = new_value
    my_object.save()

2. Use update_fields to bypass validation

If you're only updating specific fields and want to bypass the form validation:

PYTHONCopied
my_object.my_field = new_value
my_object.save(update_fields=['my_field'])

3. Handle the constraint at the database level with deferred constraints

Some databases support deferring constraint checks until the end of a transaction:

PYTHONCopied
from django.db import models

class MyModel(models.Model):
    my_field = models.CharField(max_length=100)
    
    class Meta:
        constraints = [
            models.UniqueConstraint(
                fields=['my_field'], 
                name='unique_my_field',
                deferrable=models.Deferrable.DEFERRED
            )
        ]

This is only supported in PostgreSQL and requires Django 3.1+.

The form validation is working correctly in your first case, but when you're trying to delete and save in a way that temporarily violates the constraint, the database constraint is checked before the deletion is processed, resulting in the IntegrityError.