django order_by值未存储在数据库中

Question

I have a model, Rating , which looks something like this:

class Rating(models.Model):
    upvotes = models.IntegerField(default=0)
    downvotes = models.IntegerField(default=0)
    @property
    def total(self):
        return self.upvotes - self.downvotes

I have a group of ratings which I need to order by their total property. As you can see, it is not a value stored in the database. rating_set.order_by("total") does not work.

I could use Python's sorted function on the list of ratings, with key=lambda r: r.total . But this seems to me that it would be rather inefficient and there probably exists a better way from within Django.

Does Django support ordering by derived values? Would I improve performance by making total a value stored in the database?

Answer 1

This is because the database has no idea about the total property. It only exists in python. You can replicate the same behavior by using extra to have it calculated and sorted by the database. Like so:

Rating.objects.extra(select={'total': "upvotes - downvotes"}).order_by('-total')