I'm trying to reduce a SQL call to a MySQL database and I'm curious if
Django tries to do some of what I'm doing or if my method makes sense.
I'm making a ratings app to rate ideas (a 1 to 5 star type of thing).
In my template I want to list both the number of ratings and also the
average rating. So I've made those both methods of the idea model that
I ented to call in my template.
I store the ratings so both calls don't run similar queries:
cached_ratings = None
# Methods related to ratings
#
def get_all_ratings(self):
self.cached_ratings =
Rating.objects.filter(content_type=self.get_content_type(),
object_id=self.id)
def get_num_ratings(self):
if not self.cached_ratings:
self.get_all_ratings()
return len(self.cached_ratings)
def get_average_rating(self):
if not self.cached_ratings:
self.get_all_ratings()
sum = 0
for rating in self.cached_ratings:
sum += rating.rating
return float(sum) / self.get_num_ratings()
Is this a reasonable thing to do?
I know SQL is optimized for SELECT COUNT(*) so if there were a template
where I were showing the total count and not the average I'm generally
curious what the difference in speed would be between doing a len() and
just calling the count() on the QuerySet.
Thanks,
Rob
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