title: Rails model validation on create and update only
body: <p>If I want to have validation only on create, then I can do</p>
  
  <pre class="lang-rb prettyprint-override"><code>validates_presence_of :password, :on =&gt; :create
  </code></pre>
  
  <p>But how do I say on create and update? I tried this but it didn't work:</p>
  
  <pre class="lang-rb prettyprint-override"><code>validates_presence_of :password, :on =&gt; [ :create, :update ]
  </code></pre>
  
  <p>Do I have to define the validation two times?</p>