title: Query in Rails takes far more time that in MongoDB
body: <p>I'm a little desperate. Query directly using a client as Robomongo goes smothly, but doing it in Rails takes very long (about 5 minutes). Apparently is the same query. </p>
  
  <p>The query in MongoDB:</p>
  
  <pre><code>db.items.find({_slugs: {$in: ["gang-leader-for-a-day-a-rogue-sociologist-takes-to-the-streets"]}})
  </code></pre>
  
  <p>The query in Rails:</p>
  
  <pre><code>Item.in(_slugs: "gang-leader-for-a-day-a-rogue-sociologist-takes-to-the-streets").first
  </code></pre>
  
  <p>Both queries are executed on the same database and '_slugs' is indexed.</p>
  
  <p>I would appreciate any help or hints.</p>