title: How to change validation error messages for associations?
body: <p>I have a <code>Document</code> class, with <code>has_many :fields</code>. Each Field object has a <code>name</code> attribute.</p>
  
  <p>When I validate the Document and the associated Fields, I get the following validation error message for each invalid Field association:</p>
  
  <pre><code>Fields is invalid.
  </code></pre>
  
  <p>This is a very useless error message. Instead, I want it to say:</p>
  
  <pre><code>Field '&lt;value of the name attribute&gt;' is invalid.
  </code></pre>
  
  <p>For example:</p>
  
  <pre><code>Field 'subject' is invalid.
  Field 'date' is invalid.
  </code></pre>
  
  <p>How can I do that?</p>