Re: How to aggregate on insert?

> Den 24. maj 2016 kl. 01.11 skrev James Schneider <jrschneider83@gmail.com>:
> On Mon, May 23, 2016 at 12:58 PM, Erik Cederstrand <erik+lists@cederstrand.dk> wrote:
>
> I have inherited a legacy Item model that has a composite unique key consisting of a Customer ID and a per-customer, incrementing Item ID. Assume I can't change the model.
>
> On inserts, the legacy code would let the database increment the Item ID to avoid race conditions. Something like this:
>
> INSERT INTO item_table (customer_id, item_id, name, ...) VALUES (123, (SELECT MAX(item_id) FROM item_table WHERE customer_id =123) + 1, 'MyItem', ...);
>
> Is there any way I can do the same using the Django ORM without opening up for race conditions? I.e. something better than:
>
> i = Item(customer_id=123, name='MyItem', ...)
> i.item_id = Item.objects.filter(customer_id=123).aggregate(Max('item_id'))['item_id__max'] + 1
> i.save()
>
>
> I feel like an explicit transaction wrapping this set of queries would be the way to go to help avoid (or at least detect) a race condition:

Thank you for your suggestions! The default isolation level for PostgreSQL is READ COMMITTED, so another thread could insert an identical (customer_id, item_id) pair between the select and the insert. I don't think a transaction would help here. I have a unique index on (customer_id, item_id) so save() would fail, and I only have one write statement, so there's nothing else to rollback if save() fails.

After digging around a bit more in the documentation and Django codebase, this solution seems to work for me:


from django.db.models import Value
from .models import Customer, Item


class IncrementingValue(Value):
def __init__(self, field, *args, **kwargs):
super().__init__(*args, **kwargs)
self.f = field

def as_sql(self, compiler, connection):
return '(SELECT MAX(%s) FROM %s WHERE customer_id=%%s) + 1' % (self.f.get_attname_column()[1], self.f.model._meta.db_table), [self.value.id]


c = Customer.objects.get(id=123)
i = Item.objects.create(customer=c, name='MyItem', item_id=IncrementingValue(value=c, field=Item._meta.get_field('item_id')))
i.refresh_from_db()


I didn't audit thoroughly for SQL injections. I would have used the ORM to generate the SQL for the SELECT MAX, but I can't seem to get the SQL for an aggregate() query.


Thanks,
Erik

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