title: Validate non-model field
body: <p>I've added an extra field to my <strong>new</strong> form:</p>
  
  <pre><code>&lt;%= select_tag :quantity, options_for_select(["Select a Value"].concat((1..10).to_a)) %&gt;
  </code></pre>
  
  <p>It specifies the number of copies of the record to be created.</p>
  
  <p>How can I validate the presence (or numericality) of that field, as it's not part of the model itself?</p>
  
  <p><code>validates_presence_of :quantity</code>   <strong>fails!!!</strong></p>