Sorting objects according to a field from a foreign field

I have an issue sorting objects.
First, let me explain the models i'm using:

class Call(models.Model):
...
priority = models.ForeignKey(Priority)

class Priority(models.Model):
title=models.CharField(max_length=30)
description=models.CharField(max_length=255)
weight=models.IntegerField()

I want to sort the calls according to their priority and more exactly the weight assigned
to priority.

I haven't found a way to do that.
I would need something like this:
c=Call.objects.all().order_by('-priority.weight')

Or can i specify the way the Priority class should be sorted?

Thanks,
Benedict

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