_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")

forms.py

class JobForm(forms.ModelForm):
    """
    A form used for creating a Job. It handles the start_time and end_time defaults.
    """

    class Meta:
        fields = []   # Everything except 'slug' and 'author'.
        model = Job

    def save(self, *args, **kw):
        last_entry = self.instance.author.job_set.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)

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(request, 'job/job_form.html', {'form': form})