Django: order_by API json value or cached objects - json

Question

I've got a model method that calculates the score of votes using the HN ranking algorithm in my model that also caches each score. calculate_score() is the main thing to focus on here.
class Submission(models.Model):
submission_type = models.CharField(_('Submission Type'),
max_length=255,
choices=tuple([(media.name.lower(), media.value) for media in MediaTypes]),
blank=False)
title = models.CharField(_('Title'), max_length=100, blank=False)
url = models.URLField(_('Link'), blank=False)
description = models.TextField(_('Description'))
flagged = models.BooleanField(_('Is Flagged for Review'), default=False)
user = models.ForeignKey(User, related_name='user_submissions')
thumbnail = models.ImageField()
date_submitted = models.DateTimeField(default=timezone.now)
by_votes = VoteCountManager()
objects = models.Manager()
def __str__(self):
return self.submission_type + ': ' + self.title
def get_absolute_url(self):
return reverse('submit_detail', args=[
str(self.submission_type),
str(self.id),
])
def get_vote(self):
"""
Returns the number of votes associated with a particular submission
:return: int
"""
return self.submission_votes.count()
def calculate_score(self):
"""
This is a variation of the HN ranking algorithm
:return: score
"""
secs_in_hour = float(60 * 60)
g = 1.2
delta = timezone.now() - self.date_submitted
item_hour_age = delta.total_seconds() / secs_in_hour
votes = self.submission_votes.count() - 1
score = votes / pow((item_hour_age + 2), g)
cached = cache.get('s{0}'.format(self.pk))
if not cached:
cache.set('s{0}'.format(self.pk), score, 30)
return score
I'm using the djangorestframework that serializes my model:
class SubmissionSerializer(serializers.HyperlinkedModelSerializer):
score = serializers.SerializerMethodField('rank')
class Meta:
model = Submission
def rank(self, obj):
return obj.calculate_score()
I have two ways that I think I can solve my problem, but I don't know how to do either of them, and I'm not sure which one is best. Since I'm caching the score for each individual submission. I was trying to order the submissions in a ListView like this pulling from the cache:
class SubmissionList(ListView):
model = Submission
def get_queryset(self):
return super(SubmissionHotList, self).get_queryset().annotate(
score=cache.get('s{0}'.format(x) for x in Submission.pk)
votes=models.Count('submission_votes'),
).order_by(
'-score', '-votes',
)
But I found that order_by only works on the database level, and I would have to create a database field for the calculated score, which I would like to avoid if possible. My other possibility is using the serialized data in my API for Submission in this ListView, but I'm not sure if APIs are only there for external applications, or if I could use them in the same application that the API is generated.
I guess my question is. Would it be better to get the cached objects to list each submission in a particular order, or could I use the API to accomplish this? And if I used the API, how could I parse and order the JSON data in the ListView, while keeping votes as a secondary ordering mechanism?

Answer 1

if you want to display all items without pagination, you will not lose any performance if you do something like
import operator
class SubmissionList(TemplateView):
template_name = "submissions.html"
def get_context_data(self, **kwargs):
context = super(SubmissionList, self).get_context_data(**kwargs)
submissions = Submission.objects.all().annotate(
votes=models.Count('submission_votes')
)
sorted_submissions = sorted(submissions, key=lambda x:(cache.get('s{0}'.format(x.pk)), x.votes), reverse=True)
context['submissions'] = sorted_submissions
return context