title: Manipulating subdocuments
body: <p>I have a structure where I need to add or subtract a specific value from a subdocument array, however when I use <code>$set</code> all the other properties gets removed if I do not explicitly set a value for them. How can I query the subdocument I need and edit the property value leaving the other objects &amp; properties intact?
  example</p>
  
  <pre><code>{
     title: 'james',
     subby:[{id:1323313,total:10,name:'test'}, 
            {id:2222222,total:10,name:'test2'}
           ]
  }
  </code></pre>
  
  <p>In this example I would like to subtract 1 from the total of the subdocument with the name 'test'</p>