Question

我正在为我的兄弟会写一个Django应用程序来投票给rushees，我正在尝试优化我的一个计算投票的查询并打印计数以及来自其应用程序的信息。 Django-debug工具栏告诉我我有很多重复的查询

（为清晰起见，下面的代码已被截断和编辑）

models.py

 votechoices = ((1, "Yes"),(2, "No"),(3, "Abstain"))

 class Vote(models.Model):
      brother = models.ForeignKey(Brother)
      rushee = models.ForeignKey(Rushee)
      choice = models.IntegerField(choices=votechoices, default=3)

 class Rushee(models.Model):
      first_name = models.CharField(max_length=40)
      last_name = models.CharField(max_length=40)
      #ETC, ETC

 class Application(models.Model):
      rushee = models.ForeignKey(Rushee)
      address = models.CharField(max_length=200)
      cellphone = models.CharField(max_length=30)
      #ETC, ETC

views.py

def getvotecount(request):

     # get all the applications ( we only vote on people who have an application)
     applicationobjs = Application.objects.select_related('rushee').all()

     # iterate through the applications and count the votes
     for app in applicationobjs.iterator():

     #>>>> This Query below is a seperate query everytime! So that means that If we have 200 rushees there are 200 queries!
          counts = Vote.objects.filter(rushee=app.rushee).values('choice').annotate(count=Count('choice'))

          votesfor = sum([x['count'] for x in counts if x['choice'] == 1])
          votesagainst = sum([x['count'] for x in counts if x['choice'] == 2])

          result = [app.rushee.first_name + ' ' + app.rushee.last_name,
                    app.address, app.cellphone,
                    str(votesfor), str(votesagainst),]

    # Uninteresting stuff below that will group together and return the results

我正在尝试在（＆gt;＆gt;＆gt;＆gt;）标记的视图中优化查询，这样我就可以返回每个rushee的投票数，而不必每次都运行单独的查询！

其他信息： sqlite后端，有比应用程序更多的rushees，我们只投票给有应用程序的rushees

Answer 1

您可以使用conditional expressions在一个查询中执行此操作：

from django.db.models import Case, IntegerField, Sum, When

rushees = Rushee.objects.annotate(
    votes_for=Sum(
        Case(
            When(vote=1, then=1),
            default=0,
            output_field=IntegerField(),
        )
    ),
    votes_against=Sum(
        Case(
            When(vote=2, then=1),
            default=0,
            output_field=IntegerField(),
        )
    )
)

生成的查询集中的rushees将具有votes_for和votes_against属性，其中包含每个属性的计数。这假设没有针对没有申请的rushees记录的投票 - 但如果有的话，你可以轻松过滤掉那些。

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