Displaying both slug and ID in URL, but route by ID only in Django

Question

What I'm trying to achieve is: my News app should display a slug, but only query the article by ID in the form of /news/24/this-is-the-slug

Unfortunately I'm getting a NoReverseMatch: Reverse for 'news_detail' with arguments '('',)' and keyword arguments '{}' not found. when trying to browse an article. The URL generated in the template looks correct as stated above (I can confirm this by doing a search via Haystack, which delivers the correct URL).

models.py

class News(models.Model):
    id = models.IntegerField(primary_key=True, editable=False)
    category = models.CharField(max_length=50L)
    title = models.CharField(max_length=200L)
    rss_summary = models.TextField(max_length=2000L)
    body_text = models.TextField(max_length=5000L)
    post_date = models.DateTimeField()
    prettyurl = models.SlugField(max_length=100L)

    class Meta:
        db_table = 'news'

    def __unicode__(self):
        return self.title

    def get_absolute_url(self):
        return urlresolvers.reverse('news_detail', kwargs={'pk': self.id, 'slug': self.prettyurl })

urls.py

urlpatterns = patterns(
    '',
    url(
        r'^$',
        view=views.NewsListView.as_view(),
        name='news_index'),
    url(
        r'^(?P<pk>\d+)/',
        view=views.NewsDetailView.as_view(),
        name='news_detail'),
    url(
        r'^(?P<pk>\d+)/(?P<slug>[-\w]+)/$',
        view=views.NewsDetailView.as_view(),
        name='news_detail'),
    url(
        r'^archive/$',
        view=views.NewsArchiveIndexView.as_view(),
        name="archive_month"),
    [... more unrelated urls ...]

views.py

class NewsDetailView(DetailView):
    #name='news_detail'),
    model = News
    context_object_name = 'news'
    #slug_url_kwarg = 'prettyurl'
    #slug_field = 'prettyurl'
    template_name = 'news/detail.html'

Template

`<p><a href="{% url 'news_detail' news.slug %}">Permalink</a> for this article.`

Answer 1

Thanks @Daniel Roseman and @yuvi. With your help I managed to solve my problem by defining the URL pattern to this:

r'^(?P<pk>\d+)(?:/(?P<slug>[\w\d-]+))?/$',

Which allows all my wanted forms of

• news/nn
• news/nn/
• news/nn/a-slug
• news/nn/a-slug/

In the template, I use

{% url 'news_detail' news.id news.prettyurl %}

Which shows the fourth version in the listing above.

Thanks again!