Relationships Among These Models In Django

I want to build a website where: there is a set of 10 Cards on each Card, there are Important/Not_Important/Unknown attributes each User chooses which card are important and not i

Solution 1:

I guess I have found the solution to my question: The models would be like this :

class Card(models.Model):
    card_number = models.IntegerField(unique=True)
    content = models.TextField(null=False)

class Choice(models.Model):
    user= models.ForeignKey(User)
    card = models.ForeignKey(Card)
    is_important = models.NullBooleanField(default=None)

    class Meta:
        unique_together = ('user','card')

The key to the answer is to use unique_together Meta option.

Please review my solution and if you have a better solution or any alternative solution, please feel free to suggest. I can learn a lot from yours.

Thanks!