title: carrierwave - rails 3.1- undefined method: image_will_change
body: <p>I get an error that look like this:</p>
  
  <pre><code>undefined method `post_image_will_change!' for #&lt;Post:0xf4e9184&gt;
  app/controllers/posts_controller.rb:43:in `new'
  app/controllers/posts_controller.rb:43:in `create'
  </code></pre>
  
  <p>I've included this in my "post" model:</p>
  
  <pre><code> attr_accessible :title, :name, :content, :post_image
   mount_uploader :post_image, PostImageUploader
  </code></pre>
  
  <p>and in _form.html.erb I've added: </p>
  
  <pre><code> :html =&gt;  { :multipart =&gt; true }
  </code></pre>
  
  <p>I looked <a href="https://stackoverflow.com/questions/6822498/carrierwave-error">CarrierWave Error</a> but that doesn't help me. </p>
  
  <p>Any clues of what generates that error? I've migrated the database and so forth (followed the railscasts guide on carrierwave exactly..)</p>