title: How to enforce select a value in selection option using simple_form on Rails 3
body: <p>How could I enforce user to choose a service_type,</p>
  
  <p>because when the page loaded, the select option shows "choose one?" by default</p>
  
  <pre><code>-service_type=[a,b,c]
  = f.input :service_type, label: "service", collection: @service_type, prompt: "choose one?"
  </code></pre>
  
  <p>Thanks~</p>