Question

I have a simple Python/Django class:

class myModel(models.Model):
    date = models.DateTimeField()
    value = models.IntegerField()

and I want to get two elements from my database. First is the newest element and the second is newest positive element. So I can do this like this:

myModel.objects.all().order_by('-date')[:1][0]
myModel.objects.filter(value__gte = 0).order_by('-date')[:1][0]

Note those [:1][0] at the end – this is because I want to get maximum use of database sql engine. The thing is that I still need two queries and I want to combine it into a single one (something like [:2] at the end which will produce the result I want). I know about Django’s Q, but can’t figure out how to use it in this context. Maybe some raw sql? I’m waiting for ideas. 🙂

Answer 1

This looks like premature optimisation to me. Is two queries instead of one really so bad? At the moment, anyone who knows the Django ORM can understand your two queries. After you’ve replaced it with some funky raw SQL, that might not be the case.

You should use [0] instead of [:1][0]. Django knows how to slice querysets efficiently — both queries will result in the exact same SQL.