title: find_or_create_by with multiple conditions
body: <p>I have a DB of individuals:</p>
  
  <pre><code>First Name
  Surname
  DOB
  </code></pre>
  
  <p>I want to avoid duplicates when adding new data. I think I need to use <code>find_or_create_by</code>. Sometimes, <code>DOB</code> is not available. I want to check all three, but if <code>DOB</code> is not available (i.e. is null), I want to check just <code>Surname</code> and <code>First Name</code>.</p>
  
  <p>Is it possible?</p>