Django multiple queries with foreign keys

问题

Let's say I have two different apps :

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)

I want to get all the teachers with classes and all classes attached.

The result I want:

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

}

Is this possible to do? This is what I am currently doing:

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)

In the above code, there are four queries made with a loop which certainly increases the time complexity. Is there a better solution to this? Thanks in advance.

回答1:

Use 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
]

This will only trigger 2 db queries regardless of how many teachers and classes there are.

来源：https://stackoverflow.com/questions/52734174/django-multiple-queries-with-foreign-keys