title: Set non-database attribute for rails model without `attr_accessor`
body: <p>In PHP, I can set an attribute (that is not a column in database) to a model. E.g.(PHP code), </p>
  
  <pre><code>$user = new User;
  $user-&gt;flag = true;
  </code></pre>
  
  <p>But in rails, when I set any attribute that doesn't exist in database, it will throw an error <code>undefined method flag</code>. There is <code>attr_accessor</code> method, but what will happen if I need about ten temp attributes?</p>