Django UNIQUE constraint failed: webapp_post.slug

Question

When I'm trying to create a new Post object, Python spews out the following error:

UNIQUE constraint failed: webapp_post.slug

Here is my models.py

class Post(models.Model):
    title = models.CharField(max_length=200)
    slug = models.SlugField(unique=True)
    text = models.TextField()
    created_on = models.DateTimeField(auto_now_add=True)
    up_vote = 0 # num of up votes
    down_vote = 0 #num of down votes
    vote_total = up_vote - down_vote
    author = models.ForeignKey('auth.User', null=True, blank=True)

    CHOICES = [
        ('Hardware and OS', 'Hardware and OS'),
        ('Desktops', 'Desktops'),
        ('Tablets', 'Tablets'),
        ('Phones', 'Phones'),
        ('Wearables', 'Wearables'),
        ('Windows', 'Windows'),
        ('Mac OS X', 'Mac OS X'),
        ('Linux and Unix', 'Linux and Unix'),
        ('Programming and Computer Science', 'Programming and Computer Science'),
        ('Software Development', 'Software Development'),
        ('Web Development (Front)', 'Web Development (Front)'),
        ('Web Development (Back)', 'Web Development (Back)'),
        ('Mobile Development', 'Mobile Development'),
        ('Game Development', 'Game Development'),
        ('Algorithms and Data Structures', 'Algorithms and Data Structures'),
        ('Databases', 'Databases'),
        ('IDE / Text Editors', 'IDE / Text Editors'),
        ('Tutorial', 'Tutorial'),
        ('Opinion', 'Opinion'),
        ('Miscellaneous', 'Miscellaneous')
    ]
    field = models.CharField(choices=CHOICES, max_length=200)

    def __unicode__(self):
        return self.title

    @models.permalink
    def get_absolute_url(self):
        return ('blog_post_detail', (), 
                {
                    'slug' :self.slug,
                })

    def save(self, *args, **kwargs):
        if not self.slug:
            self.slug = slugify(self.title)
        super(Post, self).save(*args, **kwargs)

Here is my views.py

@user_passes_test(lambda u: u.is_authenticated)
def add_post(request):
    form = PostForm(request.POST or None)

    if request.method == "POST":
        if form.is_valid() and request.user.is_authenticated():
            try:
                post = form.save(commit=False)
                post.author = request.user
                post.save()
                Post.objects.create(author=request.user, title=form.cleaned_data.get("title"), text=form.cleaned_data.get("text"))
                return redirect(post)
            except IntegrityError as e:
                print(e)
        else:
            print("Invalid form")
            print(form.errors)

    return render_to_response('webapp/startthread.html', 
                              { 'form': form,
                                "authenticated": request.user.is_authenticated() },
                              context_instance=RequestContext(request))

Answer 1

Django is reporting that the database will not save your Post data because the value of the slug field is already being used by another Post .

If you don't want this behaviour, do not set the unique attribute to True on Post.slug in your model. Consider though, that the slug is often used to query the database to find the associated Post , so generally you'll want it to be unique.