Generic detail view must be called with either an object pk or a slug

gistfile1.txt

#Django error :Generic detail view must be called with either an object pk or a slug
#http://computernerddiaries.wordpress.com/

class PostView(DetailView):
  template_name = 'post_detail.html'
  context_object_name = 'post'
  model = Post

class Post(models.Model):
  title = models.CharField(max_length=50, unique=True)
  slug = models.SlugField(unique=True)
  content = models.TextField()
  def __str__(self):
    return self.title
#by pk    
urlpatterns = patterns('',
    # ...
 
    url(r'^(?P<pk>\d+)/$', PostView.as_view(), name='post_detail_url'),
)
#by slug
urlpatterns = patterns('',
    # ...
 
    url(r'^(?P<slug>\w+)/$', PostView.as_view(), name='post_detail_url'),
)

#by title 
#first override the get_object method of DetailView

class PostView(DetailView):
    
    template_name = 'post_detail.html'
    context_object_name = 'post'
    model = Post
    def get_object(self):
      object = get_object_or_404(Post,title=self.kwargs['title'])
      return object
        
#then 
urlpatterns = patterns('',
    # ...
 
    url(r'^(?P<title>\w+)/$', PostView.as_view(), name='post_detail_url'),
)