django中的非规范化字段

Question

我发现很难对 django 模型中的字段进行非规范化。我有：

class AnswerSet(models.Model):
    title = models.CharField(max_length=255)
    num_answers = models.PositiveIntegerField(editable=False, default=0)
    answers = models.ManyToManyField(Answer, through='AnswerSetAnswer')
    ...

class AnswerSetAnswer(models.Model):
    answer = models.ForeignKey(Answer)
    answer_set = models.ForeignKey(AnswerSet)
    ...

我希望 num_answers 包含集合中答案数量的计数。

如果最初有 5 个答案与 AnswerSet“食物”相关联，并且我编辑了一个答案使其与 AnswerSet“颜色”相关联，我如何重新计算 AnswerSet 中与“食物”相关的答案数量？所有信号似乎只发送新数据，所以我不能只覆盖保存方法。

我尝试过使用 m2m_changed 信号，但是当我通过管理表单编辑关系时它从未被调用过。

这是我的代码：

def update_answer_set_num_answers(sender, **kwargs):
    """
    Updates the num_answers field to reflect the number of answers
    associated with this AnswerSet
    """
    instance = kwargs.get('instance', False)

    print "no instance"              # never gets here

    if not instance:
        return

    action = kwargs.get('action')

    print "action: ", action

    if (action != 'pre_remove' and action != 'pre_add' and action != 'clear'):
        return

    reverse = kwargs.get('reverse')

    if reverse:
        answer_set = instance.answer_set
    else:
        answer_set = instance.answer_set
    num_answers = AnswerSetAnswer.objects.filter(answer_set=answer_set.id).count()

    if (action == 'pre_remove'):
        num_answers -= int(kwargs.get('pk_set'))
    elif (action == 'pre_add'):
        num_answers += int(kwargs.get('pk_set'))
    elif (action == 'clear'):
        num_answers = 0

    answer_set.num_answers = num_answers

    print 'n a: ', answer_set.num_answers
    answer_set.save()


m2m_changed.connect(update_answer_set_num_answers, \
  AnswerSet.answers.through, weak=False)

Answer 1

你真的需要去规范化吗？你可以用一个简单的聚合来计算它：

from django.db.models import Count
answersets = AnswerSet.objects.all().annotate(num_answers=Count('answers')