Re: How to aggregate on insert?

Hey Erik,

What Django version are you on?

On Tuesday, May 24, 2016 at 1:28:57 AM UTC+5:30, Erik Cederstrand wrote:

Hi,

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()

Or do I just wrap that in a loop and catch IntegrityError if I don't want to use raw SQL?


Thanks,
Erik

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