How to upload a file in Django by linking it to a specific db entry?

Question

Django

Good afternoon!
There is a blog model with fields id, text, ... , docfile

docfile = models.FileField(upload_to='documents', blank=True)

I need to link a file to an existing blog entry.
It's just that the files are loaded, but I can't link them to a specific record, i.e. a new (empty) entry appears in the database which has an id and a docfile (with the correct binding to the uploaded file)
Did according to the manual https://docs.djangoproject.com/en/2.1/topics/http/...
forms.py

class FileUploadForm (forms.ModelForm):
    class Meta:
        model = blog
        fields = ('docfile',)
        docfile = forms.FileField()

view.py

def blog_upload (request, pk):
    if request.method == 'POST':
        form = FileUploadForm(request.POST, request.FILES)
        if form.is_valid():
            form.save()
            return HttpResponseRedirect('/success/url/')
    else:
        form = FileUploadForm()
    return render(request, 'upload_doc.html', {'form': form})

to the blog_upload (request, pk) function, I pass the pk (primary key) of the entry to which the file should be added.
but I don't understand how to use this information correctly.