title: Removing or overriding an ActiveRecord validation added by a superclass or mixin
body: <p>I'm using Clearance for authentication in my Rails application. The <code>Clearance::User</code> mixin adds a couple of validations to my <code>User</code> model, but there's one of these that I would like to remove or override. What is the best way of doing this?</p>
  
  <p>The validation in question is </p>
  
  <pre><code>validates_uniqueness_of :email, :case_sensitive =&gt; false
  </code></pre>
  
  <p>which in itself isn't bad, but I would need to add <code>:scope =&gt; :account_id</code>. The problem is that if I add this to my <code>User</code> model</p>
  
  <pre><code>validates_uniqueness_of :email, :scope =&gt; :account_id
  </code></pre>
  
  <p>I get <em>both</em> validations, and the one Clearance adds is more restrictive than mine, so mine has no effect. I need to make sure that only mine runs. How do I do this?</p>