title: Django: Parent categories doesn't shown in admin interface
body: <p><strong>Problem:</strong>
  Dash signs display in 'Parent category' column before I open any menu item. I get 'Parent category' items when open menu item profile(see <code>admin.py</code> below)</p>
  
  <p>Before and after opening of menu profile: <a href="https://ibb.co/fuCZHb" rel="nofollow noreferrer">picture1</a> and <a href="https://ibb.co/kPgYAw" rel="nofollow noreferrer">picture2</a> (pay attention on Parent Category column)</p>
  
  <p><strong>My <code>models.py</code>:</strong> (attention on <code>parent_id</code>)</p>
  
  <pre><code>class Menu(models.Model):
  
      cat_title = models.CharField(max_length=150, verbose_name='Category title')
      menu_title = models.CharField(max_length=150, verbose_name='Menu title')
      parent_id = models.IntegerField(blank=True, null=True, verbose_name='Parent category', choices=(('',''),))
      url = models.CharField(max_length=255, verbose_name='URL', blank=True)
      named_url = models.CharField(max_length=255, verbose_name='Named URL', blank=True)
      level = models.IntegerField(default=0, editable=False)
  </code></pre>
  
  <p><strong>My <code>admin.py</code>:</strong> ()</p>
  
  <pre><code>class MyMenu(admin.ModelAdmin):
  
      def get_choices(self):
          choices = (('',''),)
          categories = models.Menu.objects.all().values()
          for i in categories:
              choices += ((i['id'], i['cat_title']),)
          return choices
  
      def formfield_for_choice_field(self, db_field, request):
          if db_field.name == 'parent_id':
              db_field.choices = self.get_choices()
  
          return super().formfield_for_choice_field(db_field, request)
      list_display = ('cat_title', 'menu_title', 'parent_id', 'level')
      list_display_links = ('cat_title', 'menu_title')
  
      admin.site.register(models.Menu, MyMenu)
  </code></pre>
  
  <p><strong>Question:</strong> How could I rewrite my <code>admin.py</code> to show <code>parent_id</code> items without opening any menu item profile?</p>
  
  <p>I've already tried <a href="https://docs.djangoproject.com/en/1.11/ref/models/instances/#django.db.models.Model.get_FOO_display" rel="nofollow noreferrer"><code>Model.get_FOO_display()</code></a> but it doesn't work in right way. Any help would be appreciated.</p>