Python 3 & Django Rest Framework - 如何查询 M-1-M 模型？

Question

我有这些模型：

• 组织

• 学生

• 课程

• 注册

学生属于组织

学生可以注册一门或多门课程

因此，注册记录基本上由给定的课程和给定的学生组成

class Organisation(TimeStampedModel):
  objects = models.Manager()
  name = models.CharField(max_length=50)

  def __str__(self):
    return self.name

class Student(TimeStampedModel):
  objects = models.Manager()
  first_name = models.CharField(max_length=50)
  last_name = models.CharField(max_length=50)
  email = models.EmailField(unique=True)
  organisation = models.ForeignKey(to=Organisation, on_delete=models.SET_NULL, default=None, null=True)

  def __str__(self):
    return self.email

class Course(TimeStampedModel):
  objects = models.Manager()
  language = models.CharField(max_length=30)
  level = models.CharField(max_length=2)

  def __str__(self):
    return self.language + ' ' + self.level

  class Meta:
    unique_together = ("language", "level")

class Enrollment(TimeStampedModel):
  objects = models.Manager()
  course = models.ForeignKey(to=Course, on_delete=models.CASCADE, default=None, null=False)
  student = models.ForeignKey(to=Student, on_delete=models.CASCADE, default=None, null=False)
  enrolled = models.DateTimeField()
  last_booking = models.DateTimeField()
  credits_total = models.SmallIntegerField(default=10)
  credits_balance = models.DecimalField(max_digits=5, decimal_places=2)

给定一个组织 ID，我知道我可以获取单个学生的注册信息，如下所示：

o=Organisation.objects.get(id=1)
o.student_set.all()[0].enrollment_set.all()

但我如何才能检索给定组织的所有注册？

例如，如果我想计算某个组织有多少学生注册了课程？像这样的：

o.student_set.enrollment_set.count()

Answer 1

你反向查询，用：

Enrollment.objects.filter(<b>student__organisation=o</b>).count()

甚至没有先获取Organization:

Enrollment.objects.filter(<b>student__organisation__id=1</b>).count()