having access to last (if any) end_time in every new entry of a model.

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")



#########################Problem im solving: im trying to set the begin_time to be the same as the end_time of the previous entry of this model by the same user if any (only the very first time wont have a previous end_time). 
###Here is my solution i have so far: 

#######################The view #########

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


######################the JobForm #########

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

    def __init__(self,*args, **kw):
        super(JobForm,self).__init__( *args, **kw)

    def save(self, *args, **kw):
        instance = super(JobForm,self).save()
        #last_entry = Job.objects.filter(author__id=user.id).order_by('-id')[0] 
        last_entry = Job.objects.filter(author=self.request.user).aggregate(max_end_time=Max('end_time'))['max_end_time']
        if last_entry:
            instance.start_time = last_entry.end_time
        else:
            # this is the very first record do nothing
            pass
        instance.save()
        return instance