Python 注释中的参数为什么以及如何相互影响？

我一直在读很多书，但我还是错过了一些东西

使用Django 1.11，假设我有以下模型：

class School(models.Model):
    pass

class Classroom(models.Model):
    school = models.ForeignKey(School, on_delete=models.PROTECT)
    active = models.BooleanField()
    busy = models.BooleanField()

class Chalkboard(models.Model):
    classroom = models.ForeignKey(Classroom, on_delete=models.PROTECT)

class Whiteboard(models.Model):
    classroom = models.ForeignKey(Classroom, on_delete=models.PROTECT)

我创建了一个学校，有一个教室，有两块白板和两块黑板：

s = School()
s.save()

c = Classroom(school=s, active=True, busy=False)
c.save()

Chalkboard(classroom=c).save()
Chalkboard(classroom=c).save()

Whiteboard(classroom=c).save()
Whiteboard(classroom=c).save()
Whiteboard(classroom=c).save()

我想总结一下每所学校有多少黑板是活跃的，但并不繁忙

q = School.objects.filter(
    Q(classroom__active=True) & Q(classroom__busy=False)
).annotate(
    chalkboard_count=Count('classroom__chalkboard'),
)

q[0].chalkboard_count
2                        # as expected

现在我想知道黑板和白板

q = School.objects.filter(
    Q(classroom__active=True) & Q(classroom__busy=False)
).annotate(
    chalkboard_count=Count('classroom__chalkboard'),
    whiteboard_count=Count('classroom__whiteboard'), # added this line
)

q[0].chalkboard_count
6                        # expected 2
q[0].whiteboard_count
6                        # expected 3

如果我将调用链接到annotate，我会得到相同的结果

q = School.objects.filter(
    Q(classroom__active=True) & Q(classroom__busy=False)
).annotate(
    chalkboard_count=Count('classroom__chalkboard')
).annotate(
    whiteboard_count=Count('classroom__whiteboard')
)

q[0].chalkboard_count
6                        # expected 2
q[0].whiteboard_count
6                        # expected 3

一直以来，计数都是我所期望的

Chalkboard.objects.count()
2
Whiteboard.objects.count()
3

---

我做错了什么？

来自您发布的链接：

组合多个聚合 将多个聚合与annotate（）组合将产生错误的结果 由于使用联接而不是子查询而导致的结果： 但是，对于大多数骨料，无法避免此问题， 计数聚合有一个独特的参数，可以帮助：

---

它就在我面前，我错过了。谢谢！

Book.objects.annotate(
    Count('authors', distinct=True), 
    Count('store', distinct=True)
)