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!

Solution 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.

Solution 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.

Comments

[karthikr]

I think you are getting confused with manytomany. You would just do =

[Martijn Pieters]

Why not relate directly to the user? Or did you want to create multiple watchlists per user?

[Nick B]

If ultimately Im trying to query to get all the Things that are being watched by a particular User, should I set up my models differently? Would it be better to place a ForeignKey from WatchList to Thing, rather than the way I described above? The ultimate goal is to query to find all things being watched by a particular user...

[Nick B]

Hmm okay that makes a little more sense. How would I then query to find all items that a user is watching?

[SColvin]

explanation added to my answer.

[Nick B]

Ok that makes sense, but I receive an AttributeError: 'User' object has no attribute 'watchlist'. Any ideas why?

[Nick B]

Ok thanks for your input but I am still pretty confused. Maybe I have set up my models wrong. If I want to be able to query to get all Things that a User is Watching, should I use a ManyToMany relationship instead? I also want to be able to get all Users that are Watching a certain Thing.. Sorry for the confusion on my part, but I appreciate your ideas!

[Martijn Pieters]

Your user object has a .watchlist_set manager; user.watchlist_set.get().thing_set.all() would be all things a user is watching.

[SColvin]

the related name was 'watchlists' not 'watchlist'

[Nick B]

I put that exact code into my python interpreter. With the models defined above, this code: >>> user.watchlists results in AttributeError: 'User' object has no attribute 'watchlists'. The same thing for user.watchlist. What am I missing? Thanks for any ideas!

[Nick B]

Ok well now Im super confused now. Trying z.watchlist = WatchList.objects.filter(user__username='SomeDude') has resulted in a ValueError: Cannot assign "[<WatchList: WatchList object>, <WatchList: WatchList object>, <WatchList: WatchList object>, <WatchList: WatchList object>]": "Thing.watchlist" must be a "WatchList" instance.

[Martijn Pieters]

@NickB: That is a list of WatchList objects, your user has multiple. Get just one instead; WatchList.objects.filter(user__username='SomeDude')[0] would be the first matching watchlist.

[Nick B]

Ok well I think I may have a problem with the way I setup my models, in that case. What I am hoping to achieve is to query for: Thing.objects.filter(watchlist__user__username='SomeDude') so that I can get all things that a user is watching. Have I set up my models incorrectly in that case? Sorry for the confusion on my part..

[Nick B]

Oh wait actually I just realized that that query worked! Sometimes I guess stumbling through a problem leads to the answer! Thanks for your assistance, apologies for my oversight!

[Martijn Pieters]

Glad to have been of help!