title: Integrity error in django form saving
body: <p>Hey folks I have my model like :</p>
  
  <pre><code>class Rule(models.Model):
      Ruleinfo = models.CharField(max_length=5,null=False)
      Ispname  = models.CharField(max_length=5,null=False)
      priority = models.ForeignKey('Priority',related_name ="priority1")
      From  =    models.IPAddressField(null=True)
      To = models.IPAddressField(null=True)
      def __unicode__(self):
          return u'%s %s %s %s %s %s %s %d' % (self.Ruleinfo, self.Ispname, self.priority,
                                        self.From, self.To)
  
  
  class Priority(models.Model):  
      priority =  models.IntegerField(null = True)
      ispname = models.ForeignKey('Rule' ,related_name="ispname1")
      rule = models.ForeignKey('Rule',related_name="rule1")
  </code></pre>
  
  <p>and here is my forms</p>
  
  <pre><code>class RuleInfoForm(ModelForm):
       Ruleinfo = forms.CharField(max_length=5)   
       Ispname  = forms.CharField(max_length=5)
       priority = forms.IntegerField()
       From  =       forms.IPAddressField()   
       To    =       forms.IPAddressField()
       class Meta:
          model = Rule
          fields = ("Ruleinfo","Ispname","priority","From","To") 
  </code></pre>
  
  <p>I am saving this form in my views as follow </p>
  
  <pre><code>def multiwanrule_info(request):
      data = {}
      try:
         form = RuleInfoForm(request.POST)
      except:
          pass
      if form.is_valid():
          rl_frm = form.save(commit=False)
          rl_frm.save()
      else:
  
          form = RuleInfoForm() 
          data['form'] = form    
      return render_to_response('networking.html',data)
  </code></pre>
  
  <p>But i am getting integrity error i.e </p>
  
  <pre><code>networking_rule.priority_id may not be NULL
  </code></pre>
  
  <p>why i am getting this error? Why it is not taking automatically like other tables </p>