title: Get objects for which a type of foreign key exists
body: <p>I'm using <a href="https://github.com/alex/django-taggit" rel="noreferrer">django-taggit</a> to create an app that stores not only to-do items but also informational items.  Both a to-do items and informational items can be tagged.</p>
  
  <p>When I pull a list of tags for to-do items, I run the following query:</p>
  
  <pre><code>action_tags = Tag.objects.order_by('name').filter(action__complete=False).annotate(action_count=Count('action'))
  </code></pre>
  
  <p>This gives me the name of all tags for which there are incomplete to-dos.  It also gives me the count of incomplete to-dos.</p>
  
  <p>For the informational items, there is no field for "complete"; information items just "are".  So I want to write a query that pulls all the tags for which there is at least one informational item.  How might that be written?</p>