Question

I would like to create movies database, where user will be able to mark movies he/she watched and liked:

class Movies(ndb.Model):
    watched = ndb.UserProperty()
    liked = ndb.UserProperty()

Will that work? I use Google accounts.
How should I choose later all movies user liked?

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Upd. I’ve followed systempuntoout approach and use the following code to save user choices:

user = users.get_current_user()
if user:
    userschoices = models.UsersChoices(
        movie=ndb.Key(models.Movies, movie_id), # TODO: what if movie_id is wrong?
        watched=True,
        user_id=user.user_id()
        )
    try:
        userschoices.put()
        self.response.out.write('1')
    except:
        self.response.out.write('0')

But if user makes his choice several times, then several records are added to the datastore…
Wouldn’t be it better just to save user id and movie id as keyname?

userschoices = models.UsersChoices.get_by_id(user.user_id() + '-' + movie_id)
if userschoices is None:
    userschoices = models.UsersChoices(id=user.user_id() + '-' + movie_id)
userschoices.movie = ndb.Key(models.Movies, movie_id) # TODO: what if movie_id is wrong?
userschoices.user_id = user.user_id()
if option == 'liked':
    userschoices.liked = True
elif option == 'watched':
    userschoices.watched = True

However, with such approach if I don’t pass liked, then it overwrites its value with None (the same with watched, if not passed, None is used).

Answer 1

I would go with two different Models, one that stores all the Movies details and one to store the UserChoices :

class Movies(ndb.Model):
    title = ndb.StringProperty(required=True)
    director = ndb.StringProperty()
    whatever = ndb.StringProperty()

class UsersChoices(ndb.Model):
    movie = ndb.KeyProperty(kind=Movies, required=True)
    watched = ndb.BooleanProperty(required=True)
    liked = ndb.BooleanProperty(required=True)
    user_id = ndb.StringProperty(required=True)

    @classmethod
    def get_liked_movies(cls, user_id):
        return cls.query(cls.user_id == user_id, cls.liked == true).fetch(10)

    @classmethod
    def get_watched_movies(cls, user_id):
        return cls.query(cls.user_id == user_id, cls.watched == true).fetch(10)

    @classmethod
    def get_by(cls, user_id, movie_key):
        return cls.query(cls.user_id == user_id, cls.movie == movie_key).get()

If you need to store informations about users you should create your UserInfo Model, keyed by user_id from the users API, with all the details Properties your application needs.

class UserInfo(ndb.Model):
        #Keyed by user_id 
        nickname = ndb.StringProperty()
        email = ndb.StringProperty()

To create a new UserInfo, you could do:

from google.appengine.api import users

user = users.get_current_user()
userinfo = UserInfo(
        id = user.user_id(),
        nickname = user.keyname(),
        email = user.email()
      )
userinfo.put()

Then, when the user is logged in, use his/her user_id to retrieve the watched/liked movies.

from google.appengine.api import users

user = users.get_current_user()
userinfo = ndb.Key(UserInfo, user.user_id()).get()
watched_movies = UsersChoices.get_watched_movies(userinfo.key.id())
liked_movies = UsersChoices.get_liked_movies(userinfo.key.id())