title: Annotate a comma-separated list of related items onto Django queryset
body: <p>On the face of it, this might seem like a weird thing to request but I need a comma-separated string of related items for a model. Taking the Author/Book model example from any tutorial, here's what I'm currently doing:</p>
  
  <pre><code>authors = Authors.objects.all().prefetch_related('books')
  for author in authors:
      author.book_titles = ', '.join([book.title for book in author.books.all()])
  </code></pre>
  
  <p>It's by no means heavy, but it feels redundant. Like the database could be doing it. In an ideal world, I feel like I should able to annotate this on with one of these fancy new  <a href="https://docs.djangoproject.com/en/1.9/ref/models/database-functions/" rel="noreferrer">database functions</a>. Here's a fantasy example, using a made-up function called <code>Joiner(..)</code>:</p>
  
  <pre><code>Authors.objects.annotate(book_titles=Joiner('books__title', separator=', ')
  </code></pre>
  
  <p>Is that possible? If so, how?</p>