title: get value of field from other form django admin
body: <p>simply i want to build a small email application between users(admins) in django admin i was successful to make users send and receive messages, in the next step: users should be able to reply to received messages. 
  simply i added a "Reply" button in indox form to redirect it to outbox form, the problem now is how can i add the to:mail in the request?</p>
  
  <p>my <strong>model.py</strong></p>
  
  <pre><code>class Mail(models.Model):
  from_user = models.ForeignKey(settings.AUTH_USER_MODEL,
                                verbose_name=_('From User'),
                                related_name='sent_mails')
  to_user = models.ForeignKey(settings.AUTH_USER_MODEL,
                              verbose_name=_('To User'),
                              related_name='recieved_mails')
  subject = models.CharField(_('Subject'), max_length=255)
  body = models.TextField(_('Body'))
  read = models.BooleanField(default=False)
  
  created_at = models.DateTimeField(auto_now_add=True)
  last_update = models.DateTimeField(auto_now=True)
  </code></pre>
  
  <p><strong>admin.py</strong> </p>
  
  <pre><code>class Inbox(Mail):
  
       class Meta:
           app_label = 'internal_mails'
           proxy = True
           verbose_name = _("Inbox")
           verbose_name_plural = _("Inbox")
  
  
  class Outbox(Mail):
  
      class Meta:
          app_label = 'internal_mails'
          proxy = True
          verbose_name = _("Outbox")
          verbose_name_plural = _("Outbox")
  
  class InboxAdmin(MailAdmin):
      list_display = ('id', 'from_user', 'subject',)
      exclude = ('read',)
  
      def get_queryset(self, request):
         queryset = super(InboxAdmin, self).get_queryset(request)
         return queryset.filter(to_user=request.user)
  
      def has_add_permission(self, request, obj=None):
          return False
  
      def changeform_view(self, request, object_id=None, form_url='', extra_context=None):
          response = super(InboxAdmin, self).changeform_view(
              request, object_id=object_id,
              form_url=form_url, extra_context=extra_context
          )
          if isinstance(response, HttpResponseRedirect):
              obj = Mail.objects.get(id=object_id)
              print(obj.from_user)
              response = HttpResponseRedirect('/'.join(response.url.split('/')[:-2] + ['outbox/add/']))
          else:
             obj = Inbox.objects.get(id=object_id)
             obj.read = True
             obj.save()
          return response
  
  
  class OutboxAdmin(MailAdmin):
      list_display = ('id', 'to_user', 'subject',)
      list_filter = ('to_user',)
      exclude = ('from_user', 'read',)
  
      def get_queryset(self, request):
          queryset = super(OutboxAdmin, self).get_queryset(request)
          return queryset.filter(from_user=request.user)
  
      def save_model(self, request, obj, form, change):
          obj.from_user = request.user
          obj.save()
  
  
  admin.site.register(Inbox, InboxAdmin)
  admin.site.register(Outbox, OutboxAdmin)
  </code></pre>