Django - optimization question

If you have some models:

class Teacher(models.Model):
    name = models.CharField(max_length=50)

class Student(models.Mode开发者_运维百科l):
    age = models.PositiveIntegerField()
    teacher = models.ForeignKey(Teacher, related_name='students')

and you use it like this:

>>> student = Student.objects.get(pk=1)
>>> student.teacher.name  # This hits the database
'Some Teacher'
>>> student.teacher.name  # This doesn't (``teacher`` is cached on the object)
'Some Teacher'

That's awesome. Django caches the related object so that you can use it again without having to abuse your database.

But, if you use it like this:

>>> teacher = Teacher.objects.get(pk=1)
>>> for student in teacher.students.all():  # This hits the database
...     print(student.age)
... 
8
6
>>> for student in teacher.students.all():  # This does too (obviously)
...     print(student.age)
... 
8
6

There's no caching or efficient access to related objects this direction.

My question is thus: Is there a built-in (or non-problematic way) to backward access related objects in an efficient way (a cached way), like you can in the student.teacher example above?

The reason I want this is because I have a model with multiple methods that need access to the same related objects over and over, so a page that should have 12 queries ends up with about 30.

There isn't any built-in way. I've written about this issue on my blog, with a technique to optimise accessing reverse relationships.

orokusaki,

You just need to cache the queryset as a python variable

students = teacher.students.all()

And then just use students in your for loops.

Below is a link to Django's own documentation about this specific issue :-)

http://docs.djangoproject.com/en/1.1/topics/db/optimization/#understand-cached-attributes

Try this perhaps?

teacher = Teacher.objects.select_related().get(pk=1)

http://docs.djangoproject.com/en/dev/ref/models/querysets/#django.db.models.QuerySet.select_related

I have never used select_related in concert with a .all() on its result, so I'm not sure if it will yield DB savings or not.