title: Django annotation based on multiple columns
body: <p>So in the <a href="https://docs.djangoproject.com/en/dev/topics/db/aggregation/" rel="nofollow">docs</a> there is the following annotation example:</p>
  
  <pre><code>Book.objects.annotate(Count('authors'))
  </code></pre>
  
  <p>which then allows you to do fun things like sort depending on the amount of authors. I would like to do the following:</p>
  
  <pre><code>Example.objects.annotate(value=sum('column1','column2','column5'))
  </code></pre>
  
  <p>now of course this does not work, but it shows what I want: to add <code>value</code> which is the <code>sum</code> of the numbers in a row that are in the first, second, and fifth column.</p>
  
  <p>How can you annotate based on <strong>multiple</strong> column values in each row?</p>