title: Ruby - Validates field presence after create
body: <p>Is it possible to validate a field for presence after the initial creation?
  I want to make phone number mandatory if the user wants to update their account after signing up.</p>
  <pre><code>  validates :phone, presence: true, if: .....
  </code></pre>
  <p>if I use <code>on: :update</code> I can no longer authenticate until the field is filled</p>