title: Can not add ActiveRecord callback by using class_eval method
body: <p>I have a User model, which includes module Staff:</p>
  
  <pre><code>class User &lt; ActiveRecord::Base
    include Staff
    ...
  end
  </code></pre>
  
  <p>I want to add an after_update callback to all the models that include this module:</p>
  
  <pre><code>module Staff
    def self.included(model)
      model.class_eval do
        after_update :callback
      end
    end
  
    private
    def callback
      ...
    end
  end
  </code></pre>
  
  <p>I get a NoMethodError: </p>
  
  <pre><code>undefined method `after_update' for Object:Class
  </code></pre>
  
  <p>What am I doing wrong?</p>