Django Limit ManytoMany queryset based on selected FK

Question

I have a model that looks like this:

class Invite(models.Model):
    user = models.ForeignKey(User)
    event = models.ForeignKey(Event)
    roles = models.ManyToManyField(Role, blank=True, null=True)
    sent =  models.BooleanField("Invite Sent", default=False, editable=False)
    created = models.DateTimeField(auto_now_add=True)

    def __unicode__(self):
        return u"%s" % self.user

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


class Role(models.Model):
    """
    This associates a user's role to an event
    """
    event = models.ForeignKey(Event, related_name="roles")
    roletype = models.ForeignKey(RoleType)
    profiles = models.ManyToManyField(Profile, related_name="roles",
            blank=True, null=True)
    modified = models.DateTimeField(auto_now=True)
    created = models.DateTimeField(auto_now_add=True)

So whenever a new Event is created, a bunch of roles gets created along with it. In the Invite model, how can I only show the roles associated with the event I've selected in the change form within Django Admin instead of showing all the entries in the Role model?

Answer 1

You probably want something along the lines of:

class InviteAdmin(admin.ModelAdmin):
    def get_form(self, request, obj=None, **kwargs):
        self.instance = obj # Capture instance before the form gets generated   
        return super(InviteAdmin, self).get_form(request, obj=obj, **kwargs)

    def formfield_for_manytomany(self, db_field, request=None, **kwargs):
        if db_field.name == 'role' and self.instance:
            # restrict role queryset to those related to this instance:         
            kwargs['queryset'] = self.instance.event.roles.all()
        return super(InviteAdmin, self).formfield_for_manytomany(
            db_field, request=request, **kwargs)

Django documentation for formfield_for_manytomany