django模型manytomany过滤器

Question

Lets say I have User, Newsletter and Subscription models, Subscription is the mapping table for User and Newsletter, eg. Subscription has foreign key to User and Newsletter

class User(models.Model):
  ...

class Newsletter(models.Model):
  ...

class Subscription(models.Model):
  subscriber = models.ForeignKey(User)
  newsletter = models.ForeignKey(Newsletter)

How do I get the query set that returns Newsletter that a particular user has subscribed to in a very simple query? I feel there is one, but can't really think of it unless going through multiple statements to achieve this.

Answer 1

This should work:

Newsletter.objects.filter(
    id__in=[subscription.newsletter.id for subscription in
            Subscription.objects.filter(subscriber=user)]

More readable(and verbose) version:

subscriptions = Subscription.objects.filter(subscriber=user) # all subscriptions for a given user

newsletter_ids = [subscription.newsletter.id for subscription in subscriptions]

Newsletter.objects.filter(id__in=newsletter_ids) # answer