title: Mongoid: Querying for documents for certain fields which contains a given string
body: <p>I am a little stuck here (mongoid docs does not seem to give an answer)</p>
  
  <pre><code>Question.where(:text.contains=&gt;"perfect")
  </code></pre>
  
  <p>I would like to find questions which text field contains a given word, in this case, perfect.</p>
  
  <p>What is the correct query here, and how can i improve the performance of such queries?</p>