title: Filtering IQueryable sub list
body: <p>Working with Entity Framework, but that's probably irrelevant 
  If I have an Iqueryable, how do I filter a sub list and keep it IQueryable so it doesn't yet hit the DB?</p>
  
  <p>If I have 10 items and each has 3 sub items, how do I filter it so there's a return of all 10 items and their subitems are filtered to where id = 1?</p>
  
  <p>class Item has about 20 properties on it, so I wouldn't want to use projection on each of them, because of maintenance concerns..</p>
  
  <pre><code>items = items.select(??);//how to do this so items are returned, and their children are filtered?
  
  class SubItem
  { private int ID { get; set; }
  }
  class Item 
  {
  private List&lt;SubItem&gt; SubItems { get; set; }
  }
  </code></pre>