title: Objects with multiple key columns in realm.io
body: <p>I am writing an app using the Realm.io database that will pull data from another, server database. The server database has some tables whose primary keys are composed of more than one field. Right now I can't find a way to specify a multiple column key in realm, since the primaryKey() function only returns a String optional.</p>
  
  <p>This one works:
      //index
      override static func primaryKey() ->String?
      {
          return "login"
      }</p>
  
  <p>But what I would need looks like this:</p>
  
  <pre><code>//index
  override static func primaryKey() -&gt;[String]?
  {
      return ["key_column1","key_column2"]
  }
  </code></pre>
  
  <p>I can't find anything on the docs on how to do this.</p>