Django使用外鍵進行多個查詢

Question

假設我有兩個不同的應用程序：

teacher/models.py:

  Teacher(models.Model):
     name = models.CharField(max_length=300)


class/models.py:

  Class(models.Model):
     name = models.CharField(max_length=300)
     teacher = models.ForeignKey(Teacher)
     students = models.ManyToManyField(Student)

我想讓所有帶班授課的老師和所有班級都附上。

我想要的結果：

{[
   teacher: '3L' #Teachers Id
   classes: ['20L','14L','30L'] #list of Class objects or ids with the above teacher
],
[# similar to above]

}

這可能嗎？ 這是我目前正在做的：

classes = Class.objects.all()
teachers = Teacher.objects.filter(id__in=classes.value_list('teacher',flat=True).distinct())
for teacher in teachers:
    classes_for_teachers = classes.objects.filter(teacher=teacher)

在上面的代碼中，使用循環進行了四個查詢，這肯定會增加時間復雜度。 有更好的解決方案嗎？ 提前致謝。

Answer 1

使用prefetch_related ：

teachers = Teacher.objects.prefetch_related('class_set')

# what you want is not a valid Python structure (set of lists (looking like dicts))
# list of dicts makes more sense
result = [
    {'teacher': t.pk, 'classes': t.class_set.all()}
    for t in teachers
]

無論有多少老師和班級，這只會觸發2個數據庫查詢。