Binding a ModelForm to an existing instance

I have a ModelForm:

class ArticleForm(forms.ModelForm):

    class Meta:

        model = Article

        fields = ['url',]

The Article model has a url field which is unique. I want users to enter a URL. If the Article already exists, it gets reused. Otherwise, a new instance is created.

And the corresponding view:

 1 @login_required

 2 def get_article(request):

 3    if request.method == 'POST':

 4        # Should I get a new instance here?
 5        form = ArticleForm(request.POST, instance=None)

 6        if form.is_valid():

 7            article, created = Article.objects.get_or_create(url=form.cleaned_data['url']) 

 9            # Some more work goes here
10            return redirect(reverse('articles:list'))

11        else:

12            form = ArticleForm()

13    return render(request, 'articles/get_article.html', {'form': form})

The form is validating the uniqueness of the model, which is great. The dilemma I am having is that I would like to bind the form to the existing instance in line 4. But at that point, the form has not been cleaned, so I wouldn't want to hit the database with it.

What's the proper pattern to do this?

Thanks a lot!

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