title: Rails model validation in different context
body: <p>Let's say you have a Product model that is considered valid when is created without a price.
  But is not allowed to appear on the website without a price. </p>
  
  <p>Maybe is not the best example but just go with it.</p>
  
  <p>How would you go to implement model validation in different contexts ? 
  Other solutions beside conditional validations since the model doesn't require additional fields.</p>