Calculating average age in Django

Question

I need to find out average age of items (groupped by various criteria, but it does not seem to be the issue). However I fail to find how to effectively create aggregation over DateTimeField using Django (on top of MySQL).

Pure Item.objects.all.aggregate(Avg('created')) seems to produce absolutely bogus values (eg. 20081988007238.133) and using Item.objects.all.extra(...) I have not found a way how to aggregate.

Of course I can manually create SQL query (something like SELECT AVG(UNIX_TIMESTAMP(created)) FROM items_item), but I'd prefer to avoid using MySQL specific code in the application.

Just for the reference, sample model I use for testing:

class Item(models.Model):
    name = models.CharField(max_length = 255)
    created = models.DateTimeField()

Answer 1

I think there is no other way as using the extra method. It should look like this then (not tested):

Item.objects.extra('avg': 'AVG(UNIX_TIMESTAMP(created))'.values('avg')

The problem is, that you have to convert the date to a UNIX timestamp before. Otherwise you cannot calculate the average. An average of string will indeed produce garbage.