title: Rails: calling standard validations from a custom validator
body: <p>Is there a way to call standard Rails validators from within a custom validator?</p>
  
  <p>I have a combination of OAuth/email signup/sign in and I want to be able to call certain validators on each method of authentication. For instance, I want to be able to call <code>validates_uniqueness_of :email</code> if the user signs up through email and then call a single validator, for instance <code>validates_with UserValidator</code>.</p>
  
  <p>If there isn't a way to do this I'm just going to use state tracking and a series of <code>:if</code> validations.</p>