Django Adding Values to ForeignKey

Question

I'm trying to set up a way for users to "watch" certain items (i.e. add items to a list containing other items by other users):

class WatchList(models.Model):
    user = models.ForeignKey(User)

class Thing(models.Model):
    watchlist = models.ForeignKey(WatchList, null=True, blank=True)

How do I add a Thing to a users WatchList?

>>> from myapp.models import Thing
>>> z = get_object_or_404(Thing, pk=1)
>>> a = z.watchlist.add(user="SomeUser")

  AttributeError: 'NoneType' object has no attribute 'add'

How can I add the item to the watchlist? And/or is this the appropriate way to set up my model fields? Thanks for any ideas!

Answer 1

z.watchlist is the reference itself, it is not a relationship manager. Just assign:

z.watchlist = WatchList.objects.get(user__name='SomeUser')

Note that this assumes there is only one WatchList per user.

Answer 2

As karthikr said you may be getting confused with manytomanyfield, if you really want an intermediary model, you might have something like this:

# Models:
class WatchList(models.Model):
    user = models.ForeignKey(User, related_name='watchlists')

class Thing(models.Model):
    watchlist = models.ForeignKey(WatchList, null=True, blank=True)

# Usage:
user = User.objects.get(name='???') # or obtain the user however you like
wl = WatchList.objects.create(user=user)
thing = Thing.objects.get(id=1) # or whatever
thing.watchlist = wl
thing.save()

# get users watch lists:
user.watchlists
...

Otherwise you might want to extend the user model.