Switch score calculation to pure Python

Rip out the score expression madness, and calculate the score in Python,
since I hear that Python is better at math than SQL is.

vendor/symposion/reviews/models.py

@@ -19,20 +19,6 @@ from symposion.proposals.models import ProposalBase
 from symposion.schedule.models import Presentation
 
 
-def score_expression():
-    score = (
-        (Decimal(2) * F("plus_two") + F("plus_one")) -
-        (F("minus_one") + Decimal(2) * F("minus_two"))
-    ) / (
-        F("vote_count") * Decimal(1)
-    )
-
-    return Case(
-        When(vote_count=0, then=Value("0")),  # no divide by zero
-        default=score,
-    )
-
-
 class Votes(object):
     ABSTAIN = "0"
     PLUS_TWO = "+2"
@@ -196,7 +182,7 @@ class Review(models.Model):
             else:
                 # self is not the latest review so we just need to decrement
                 # the comment count
-                self.proposal.result.comment_count = models.F("comment_count") - 1
+                self.proposal.result.comment_count = F("comment_count") - 1
                 self.proposal.result.save()
         # in all cases we need to delete the review; let's do it!
         super(Review, self).delete()
@@ -274,6 +260,12 @@ class ProposalResult(models.Model):
             result, created = cls._default_manager.get_or_create(proposal=proposal)
             result.update_vote()
 
+    def calculate_score(self):
+        if self.vote_count == 0:
+            return 0
+        else:
+            return ((2 * self.plus_two + self.plus_one) - (2 * self.minus_two + self.minus_one)) / self.vote_count
+
     def update_vote(self, *a, **k):
         proposal = self.proposal
         self.comment_count = Review.objects.filter(proposal=proposal).count()
@@ -296,9 +288,8 @@ class ProposalResult(models.Model):
         self.minus_one = vote_count[VOTES.MINUS_ONE]
         self.minus_two = vote_count[VOTES.MINUS_TWO]
         self.vote_count = sum(i[1] for i in vote_count.items()) - self.abstain
+        self.score = self.calculate_score()
         self.save()
-        model = self.__class__
-        model._default_manager.filter(pk=self.pk).update(score=score_expression())
 
     class Meta:
         verbose_name = _("proposal_result")