title: django-mptt get_descendants for a list of nodes
body: <p>I am trying to get all <code>descendants(include_self=True)</code> not for one 
  Node, but for a list (a QuerySet) of Nodes. This should be one SQL 
  query. </p>
  
  <p>Example (that actually is not working:) </p>
  
  <pre><code>some_nodes = Node.objects.filter( ...some_condition... ) 
  some_nodes.get_descendants(include_self=True) #hopefully I would like 
  to have all possible Nodes starting from every node of "some_nodes" 
  </code></pre>
  
  <p>The only idea I have right now is to iterate through some_nodes and 
  run get_descendants() for every node - but this is terrible solution 
  (plenty of SQL queries). </p>
  
  <p>If there is no clean way to do it via Django ORM can you provide me a custom 
  SQL to run instead? Here you can make assumption that I have a list of 
  Node's pk. </p>
  
  <p>EDIT: If that could help - all of my "some_nodes" are placed in the same parent directory and have the same "level" in the tree. </p>