title: Projection with single child object
body: <p>Say I have something like:</p>
  
  <pre><code>{ "_id" : 1, "semester" : 1, "grades" : { student1: 70, student2: 85 }}
  { "_id" : 1, "semester" : 1, "grades" : { student1: 55, student2: 24 }}
  </code></pre>
  
  <p>I want to do a <code>find()</code> and in my projection I just want <code>student2</code>, which is a child object/attribute of <code>grades</code></p>
  
  <p>My naive approach:</p>
  
  <p><code>db.streets.find({dowhatever}, {_id:1, grades.student2: 1})</code></p>
  
  <p>Didn't work so then I looked into <a href="http://docs.mongodb.org/manual/reference/operator/projection/" rel="nofollow">http://docs.mongodb.org/manual/reference/operator/projection/</a> and I thought maybe <code>$slice</code> would be what I want, but doesn't seem to be.</p>