title: How to concatenate a string to a number inside a template tag in Django
body: <p>I've found a <a href="https://stackoverflow.com/questions/4386168/how-to-concatenate-strings-in-django-templates">similar question on StackOverflow</a>, but the solution doesn't seem to work for me, unless I'm doing it wrong. I have a ID number, which I'd like to append to a string in a template tag. Here's my attempt:</p>
  
  <pre><code>{% with "image-"|add:vid.the_id as image_id %}
       {# custom template tag to generate image #}
      {% image vid.teaser_thumbnail alt=vid.title id=image_id %}
  {% endwith %}
  </code></pre>
  
  <p>But <code>image_id</code> is coming out as empty.</p>
  
  <p>What am I doing wrong here?</p>
  
  <p>My desired output of <code>image_id</code> would be something like "image-8989723123".</p>