title: Add field to model, as annotate do
body: <p>For example, I have a model Posts and Comments.</p>
  
  <pre><code>Post.objects.annotate(Count('comment'))
  </code></pre>
  
  <p>I am making such query to get count of comments for every post. But in template i should check if this value greater than 4. As I know there is no such templatetag. For now I put method in model that executes comparision operation.</p>
  
  <p>The question is, how to put field to model as annotate do?</p>