title: Is find_or_create_by thread safe in Mongoid?
body: <p>I have a web app that uses Mongoid's find_or_create_by method. </p>
  
  <pre><code>poll = Poll.find_or_create_by(fields)
  </code></pre>
  
  <p>Before we go into production, I am trying to run through the failure scenarios, and it occurred to me that multiple users could try to access this resource using this method. Is there any likelihood that it could create multiple instances of the same object? What can I do to prevent that?</p>