title: Django mptt: Get all descendants of a category
body: <p>I would like my categories to show all items in that category including descendant categories, so the parent category contains all items for subcategories.</p>
  
  <p>I have tried adding this method to my Category class in models.py</p>
  
  <p><code>def get_all_products(self):
          # To display all items from all subcategories
          return Product.objects.filter(category__in=Category.objects.get_descendants(include_self=True))
  </code></p>
  
  <p>And added this to my template.html, but it does't work. What am I doing wrong?</p>
  
  <p><code>{% for product in instance.get_all_products %}
      &lt;li&gt;{{ product.product_name }}&lt;/li&gt;
  {% empty %}
       &lt;li&gt;No items&lt;/li&gt;
  {% endfor %}
  </code></p>