How to use Django to manage free spaces in a group of users?

Question

I have the following (stripped down) code:

# games/models.py
side_choices = [('A', 'Attack'), ('D', 'Defense')]
position_choices = [(0, 'Commander'), (1, 'Knight'), (2, 'Mage'), (3, 'Healer')]
class Game(models.Model):
    users = models.ManyToManyField(User, through='GameParticipation)) // User is Django's user


class GameParticipation(models.Model):
    user = models.ForeignKey(User)
    game = models.ForeignKey(Game)
    side = models.CharField(choices=side_choices)
    position = models.PositiveSmallIntegerField(choices=position_choices)

# games/forms.py
class JoinForm(forms.Form):
    side = forms.ChoiceField(choices=side_choices)
    position = forms.ChoiceField(choices=position_choices)        

And I haven't a clue how I can write the code to get a list of free spaces in a game, and pass it to the JoinForm so that users don't see options for a taken space.

Answer 1

Need more info here, but assuming you know only spaces that are taken are in the database, can probably figure out which are free.

#assuming you know the game and user
game=1
user=2

#get all used spaces
usedSpaces=GameParticipation.objects.filter(game__get=game).filter(user__get=user)
#figure out from size of board, which spaces are empty

or if you know empty spaces have null values

emptySpaces=GameParticipation.objects.filter(game__get=game).filter(user__get=user).filter(position__exact=None)