title: How to parameterize django aggregation?
body: <p>I want to do something like </p>
  
  <p><code>
  query.annotate(Count('foreign_model_relation', somefield_from_foreign_model=some_value))
  </code></p>
  
  <p>That means, i want to count, how many objects from another queryset are pointing to this object. The difference between using something like filter(in=other_queryset) is, that i would like to combine this in <em>one</em> query, to avoid generating one query per object.</p>
  
  <p>Simplified Models:</p>
  
  <ul>
  <li>Group</li>
  <li>Object
  <ul>
  <li>group (Group)</li>
  </ul></li>
  <li>Vote
  <ul>
  <li>object (Object)</li>
  <li>up (Boolean)</li>
  </ul></li>
  </ul>
  
  <p>Now i want to query the up/down count of all Objects for one Group, with one or two queries, not with one/two queries per Object.</p>