我应该在Django的save方法中的哪里使用原子事务？

Question

Is this an appropriate use of atomic transactions? Why or why not?

def save(self, **kwargs):
    try:
        with transaction.atomic:
            super(User, self).save(**kwargs)
            if self.image:
                img = Image.open(self.image.path)
                if img.height > 300 or img.width > 300:
                    output_size = (300, 300)
                    img.thumbnail(output_size)
                    img.save(self.image.path)
    except (OSError, IOError):
        self.image = None
        with transaction.atomic:
            super(User, self).save(update_fields=['image'])
        raise PValidationError('Image can`t be saved')

Answer 1

hi maybe you can use some thing like this:

class MyModel(models.Model):
    # model definition

    def save(self, *args, **kwargs):
        transaction.set_autocommit(False)
        try:
            super(MyModel, self).save(*args, **kwargs)
            # do_other_things
        except:
            transaction.rollback()
            raise
        else:
            transaction.commit()
        finally:
            transaction.set_autocommit(True)

However the better is used some thing like this:

class MyModel(models.Model):
    # model definition

    @transaction.atomic
    def save(self, *args, **kwargs):
        super(MyModel, self).save(*args, **kwargs)

please for more information check: @transaction.atomic

good luck..!!

Answer 2

I think that you don't need any of the two atomic transactions actually. The best example when atomic transaction can be used is transfering the money from one account to another, for example:

def transfer_money(source_account, destination_account, quota):
    source_account.amount_of_money -= quota
    source_account.save()
    # K-BOOM
    destination_account.amount_of_money += quota
    destination_account.save()

What would happen if something went wrong in the # K-BOOM moment? One account would have less money and the second account would NOT have more as it should.

This is why we need the whole transfer_money function to be atomic:

@transaction.atomic
def transfer_money(source_account, destination_account, quota):
    source_account.amount_of_money -= quota
    source_account.save()
    # K-BOOM
    destination_account.amount_of_money += quota
    destination_account.save()