class Country(Model):
    name = CharField()

class City(Model):
    country = ForeignKey(Country, related_name = 'cities')

class Company(Model):
    country = ForeignKey(Country, related_name = 'companies')

class Office(Model):
    company = ForeignKey(Company, related_name = 'offices')
    city = ForeignKey(City, related_name = 'offices')
    
    class Meta:
        unique_together = (("company", "city"),)

class NewOfficeForm(ModelForm):
    class Meta:
        model = Office

    def __init__(self, company, *args, **kwargs):
        super(ModelForm, self).__init__(*args, **kwargs)
        self.fields['city'].queryset = company.country.cities.all()
        self.fields['city'].empty_label = None

class NewOfficeForm(ModelForm):
    class Meta:
        model = Office

    def __init__(self, company, *args, **kwargs):
        super(ModelForm, self).__init__(*args, **kwargs)
        all_cities = company.country.cities.all()
        buf = [x.id for x in all_cities if x not in [y.city for y in company.offices.all()]]
        q_set = company.country.cities.none()
        for q_id in  buf:
            q_set = q_set.__or__(all_cities.filter(id = q_id))
        self.fields['city'].queryset = q_set
        self.fields['city'].empty_label = None