title: Django. How to annotate a object count from a related model
body: <p>lets say I have a model Comments and another model Answers. I want to query all comments but also include in the query the number of answers each comment has. I thought <code>annotate()</code> would be useful in this case, but I just don't know how to write it down. The docs write that:</p>
  
  <blockquote>
    <p>New in Django 1.8: Previous versions of Django only allowed aggregate
    functions to be used as annotations. It is now possible to annotate a
    model with all kinds of expressions.</p>
  </blockquote>
  
  <p>So. This is an example of my models:</p>
  
  <pre><code>class Comments(models.Model):
     ...
  
  class Answers(models.Model):
     comment = models.ForeignKey(Comments)
  </code></pre>
  
  <p>And this is an example query:</p>
  
  <pre><code>queryset = Comments.objects.all().annotate(...?)
  </code></pre>
  
  <p>I'm not sure how to annotate the <code>Count</code> of answers each comment has. That is, how many answers point to each comment on the FK field <code>comment</code>. Is it even possible? is there a better way? Is it better to just write a method on the manager? </p>