title: MongoDB query to return only embedded document
body: <p>assume that i have a <code>BlogPost</code> model with zero-to-many embedded <code>Comment</code> documents. can i query for and have MongoDB return <em>only</em> <code>Comment</code> objects matching my query spec?</p>
  
  <p>eg, <code>db.blog_posts.find({"comment.submitter": "some_name"})</code> returns only a list of comments.</p>
  
  <p>edit: an example:</p>
  
  <pre><code>import pymongo
  
  connection = pymongo.Connection()
  db = connection['dvds']
  
  db['dvds'].insert({'title': "The Hitchhikers Guide to the Galaxy",
                     'episodes': [{'title': "Episode 1", 'desc': "..."},
                                  {'title': "Episode 2", 'desc': "..."},
                                  {'title': "Episode 3", 'desc': "..."},
                                  {'title': "Episode 4", 'desc': "..."},
                                  {'title': "Episode 5", 'desc': "..."},
                                  {'title': "Episode 6", 'desc': "..."}]})
  
  episode = db['dvds'].find_one({'episodes.title': "Episode 1"}, 
                                fields=['episodes'])
  </code></pre>
  
  <p>in this example, <code>episode</code> is:</p>
  
  <pre><code>{u'_id': ObjectId('...'),
   u'episodes': [{u'desc': u'...', u'title': u'Episode 1'},
                 {u'desc': u'...', u'title': u'Episode 2'},
                 {u'desc': u'...', u'title': u'Episode 3'},
                 {u'desc': u'...', u'title': u'Episode 4'},
                 {u'desc': u'...', u'title': u'Episode 5'},
                 {u'desc': u'...', u'title': u'Episode 6'}]}
  </code></pre>
  
  <p>but i just want:</p>
  
  <pre><code>{u'desc': u'...', u'title': u'Episode 1'}
  </code></pre>