title: Mongo Update $set not working
body: <p>I'm trying to mass update some mongo documents.</p>
  
  <p>I'm using the query</p>
  
  <pre class="lang-javascript prettyprint-override"><code>db.articles.update(
      { 
           'categories.id': ObjectId("51cd5272222wb6zs464fa4d9"), 
           'source.importer': 'pa'
      }, 
      { 
           $set : 
               { 
                   'source.expires-at': ISODate("2014-01-01T08:39:45Z") 
               } 
      }
  )
  </code></pre>
  
  <p>This query does not update the <code>source.expires-at</code> field, however the where part of the statement works fine.</p>
  
  <p>The document structure is</p>
  
  <pre><code>{
     "_id": ObjectId("5211dc100000044707000015"),
     "categories": {
       "0": {
         "id": ObjectId("51cd5272222wb6zs464fa4d9")
      } 
    },
     "source": {
       "importer": "pa",
       "expires-at": ISODate("2013-09-18T08:49:32.0Z") 
    }
  }
  </code></pre>