title: How can I use sum?
body: <p>On my tree view I should show the total (last line) of a specific field. with a float field it works with no problem but as the field is a string it doesn't . so how can I let it work?</p>
  
  <p>old_var is a float field 
  my_variable is a Char field
  my_variable should be HH:M:S on the view </p>
  
  <pre class="lang-py prettyprint-override"><code>my_variable=str(datetime.timedelta(hours=old_var))
  </code></pre>
  
  <pre class="lang-xml prettyprint-override"><code>&lt;field name="my_variable"  sum="total"/&gt;
  </code></pre>