forms.py

class JobForm(forms.ModelForm):
    """ this handles the start_time and end_time defaults"""
    class Meta:
        exclude = ['slug','author',]
        model = Job

    def save(self, *args, **kw):
        last_entry = Job.objects.filter(author=self.request.user).aggregate(max_end_time=Max('end_time'))['max_end_time']
        if last_entry:
            self.instance.start_time = last_entry.end_time
        return super(JobForm,self).save(*args, **kw)

my integrity

################entering some data the first time to my form doesnt work, it gives this error: 


IntegrityError at /job/add/article/

(1048, "Column 'author_id' cannot be null")

Request Method:   POST
Request URL:     http://localhost:8000/job/add/article/
Django Version: 	1.4.3
Exception Type: 	IntegrityError
Exception Value: 	

(1048, "Column 'author_id' cannot be null")

views.py

@login_required
def add_job(request):
    form = JobForm(data=request.POST or None)
    if form.is_valid():
        form.instance.author = request.user
        article = form.save()
        msg = "Job saved successfully"
        messages.success(request, msg, fail_silently=True)
        return redirect(article)
    return render('job/job_form.html', { 'form': form })