Question

I have a set of document objects and label objects, and I want those two objects to be linked. It’s a typical many-to-many relationship. I have the following code:

Models.py:

class Document(models.Model):
    title = models.CharField(max_length=50, unique=True)
    title_slug = models.SlugField(max_length=50, unique=True, editable=False)
    labels = models.ManyToManyField('Label')

    def save(self, *args, **kwargs):
        self.title_slug = slugify(self.title)
        super(Document, self).save(*args, **kwargs)

class Label(models.Model):
    name = models.CharField(max_length=40, unique=True)
    slug = models.SlugField(max_length=40, unique=True, editable=False)
    
    def save(self, *args, **kwargs):
        self.slug = slugify(self.name)
        super(Document, self).save(*args, **kwargs)

Views.py:

class DocumentForm(ModelForm):
    class Meta:
        model = Document
        fields = ["title","labels"]

def upload_document(request):
    if request.method == 'POST':
        form = DocumentForm(request.POST, request.FILES)
        if form.is_valid():
            new_document = form.save()
            return HttpResponseRedirect("/thanks/")

    else:
        form = DocumentForm()

    return render_to_response('upload_page.html', {'form':form}, context_instance=RequestContext(request))

When I upload a document, it gets added to the database, however no labels are being created or associated with the document. Do I need to explicitly add something to the Document’s save() function to make this happen? Or somewhere in the Views.py file? I’d imagine it’d go something like:

• Check to see if the label that’s being added already exists
• If it doesn’t, then create a new label
• Grab both the current document_id and the new/existing label_id
• Add a record to the document_labels table (automatically created for the many-to-many relationship)

I feel like that’s pretty standard functionality that I assumed would be built in to a many-to-many relationship in django, but it doesn’t seem to be working for me so far. I’m trying to avoid reinventing the wheel here.

Answer 1

As other people said, you cannot save in one-shot Document object and its ManyToMany field, because django create "intermediatary join table", which need the Document object’s ID, which is not defined at that point.

There is a save_m2m function in ModelForm, that is supposed to be called by the Form itself, as described in the doc

However, if it doesn’t work, maybe a trick is to call save_m2m in the view function, like this:

def upload_document(request):
    if request.method == 'POST':
        form = DocumentForm(request.POST, request.FILES)
        if form.is_valid():
            new_document = form.save()
            form.save_m2m()
            return HttpResponseRedirect("/thanks/")

    else:
        form = DocumentForm()

    return render_to_response('upload_page.html', {'form':form}, context_instance=RequestContext(request))