models.py

class CardCollection(models.Model):
    user = models.OneToOneField('users.User', on_delete=models.CASCADE)

    def __str__(self):
        return f"{self.user}'s card collection"


class CardPlayer(models.Model):
    collection = models.ForeignKey('users.CardCollection')
    player = models.ForeignKey('players.Player')

    def __str__(self):
        return self.player