Trying to sort queryset

I have model like this:

class Kaart(models.Model):
    name = models.CharField(max_length=200, verbose_name="Kaardi peakiri", help_text="Sisesta kaardi pealkiri (maksimum tähemärkide arv on 38)", blank=False, null=False)
    url = models.CharField(max_length=200, blank=False, null=False, verbose_name="Asukoha URL", help_text="Täisasukoht (http://www.domeen.ee/kaart/)")
    kirjeldus = models.TextField(max_length=500, verbose_name="Kommentaar", help_text="Informatsioon / viide tegijale (mitte kohustuslik)")
开发者_如何学JAVA    date_added = models.DateField(help_text="lisamis kuupäev", verbose_name="Kuupäev", blank=False, null=False, default=datetime.date.today())
    neg = models.PositiveIntegerField(default=0)
    pos = models.PositiveIntegerField(default=0)
    def diff(self):
        return self.pos - self.neg

my view - notice the commented out part:

def index(request):
    #a = Kaart.objects.all()
    a = Kaart.objects.all().order_by('diff')
    return render_to_response('index.html', {
        'k':a,
    }, context_instance=RequestContext(request))

and template:

{% for kaart in k %}
<tr>
        <td class="tc"><span class="nr">1</span></td>
        <td>
               <a href="#">{{ kaart.name }}</a> 
               <a href="{{ kaart.url }}" id="full-url" title="k.kirjeldus">URL</a>
        </td>
        <td class="tc">{{ kaart.shortdate }}</td>
        <td class="tr">
               <span class="pos">{{ kaart.diff }}</span>
               <a href="#"><img src="hinda/img/thumbs-up.jpg" /></a>
               <a href="#"><img src="hinda/img/thumbs-down.jpg" /></a>
        </td>
</tr>
{% endfor %}

And the error i get is: Caught an exception while rendering: Cannot resolve keyword 'diff' into field. Choices are: date_added, id, kirjeldus, name, neg, pos, url.

highlighted row is

{% for kaart in k %}

interesting thing is that when i comment in the part thats commented out atm and comment out sorted queryset then everything is working fine. Whats wrong here?

how can i get this query sorted by diff?

Alan.

---

You will have to do it like this I believe:

a = Kaart.objects.all().extra(
    select = {'diff': 'pos - neg'}
).order_by('diff')

---

The order_by arguments must be resolvable by the SQL interpreter in your database. Your diff method is in Python, and so isn't available at the SQL level. Baresi's answer looks good.