Python-сообщество

class DocumentGroup(models.Model):  
    title = models.CharField(u'Название группы', max_length=255)
    number = models.CharField(u'Номер группы', max_length=3)

    def __unicode__(self):
        return u'%s %s' % (self.number, self.title)

class Document(models.Model):
    group = models.ForeignKey(DocumentGroup)
    number = models.CharField(u'Номер документа в группе', max_length=3)
    title = models.CharField(u'Название документа', max_length=255)
    
    def __unicode__(self):
        return u'%s.%s %s' % (self.group.number, self.number, self.title)

class DocumentForm(forms.Form):
    
    id = forms.IntegerField(widget=forms.HiddenInput, required=False)
    documents = forms.ModelMultipleChoiceField(queryset = Document.objects.none(), widget=CheckboxSelectMultipleGroups,
                                            required=False, label=u'Документы')
    services = forms.ModelMultipleChoiceField(queryset = Service.objects.all(), widget=forms.MultipleHiddenInput)
    categories = forms.ModelMultipleChoiceField(queryset = Category.objects.all(), widget=forms.MultipleHiddenInput)
    
    def __init__(self, data=None, files=None, auto_id='id_%s', prefix=None,
                 initial=None, error_class=forms.util.ErrorList, label_suffix=':',
                 empty_permitted=False):
        
        super(DocumentForm, self).__init__(data, files, auto_id, prefix,
                 initial, error_class, label_suffix,
                 empty_permitted)
        
        services = None
        if data:
            services = data.getlist('services')
        if initial:
            services = initial['services']
        if services:    
            self.fields[u'documents'].queryset = Document.objects.filter(service__in=services).distinct()

Document.objects.filter(service__in=services).distinct()

    def __unicode__(self):
        return u'%s.%s %s' % (self.group.number, self.number, self.title)