title: How to write active record error message for Rails 3
body: <p>I'd like to define specific error message for some field of model (e.g. User), which should override the default one (activerecord.errors.messages.invalid) defined in rails-i18n. </p>
  
  <pre><code>activerecord:
    errors:
      messages:
        invalid: "foo"
  </code></pre>
  
  <p>I have tried some structure like </p>
  
  <pre><code>activerecord:
    user:
      errors:
        email:
          messages:
            invalid: "bar"
  </code></pre>
  
  <p>But it doesn't work.</p>