title: Create a configuration with symfony2
body: <p>I'm trying to create this configuration on Symfony</p>
  
  <pre><code>media:
      medias:
          category.type.subtype:
              reference : "UUID" # What is used to identify object in DB (needed)
              service : ~ # Service name "bundle.service.name"
  
              identifiers: # If empty, no generator can be created for this media
                  channelId: # Example
                      label: ~ # Label for front
                      key: "channel_id" # Identifier in bdd (default : the name of identifier)
                  channelTitle: ~
                  # Other identifier....
  
              api: # Api configuration (can be null if no API)
                  consumer_key: ~
                  consumer_secret: ~
                  # Others parameters...
  
              parameters:
                  label: "Media 1" # Label in the front Default : generated with the name
                  page:
                      factor: 50 # Factor (default 20)
                      styles:
                          style1:
                              css_class: class1
                          style2:
                              css_class: class2
                          # Other styles...
          category.type.subtype2: #parameters.....
  </code></pre>
  
  <p>I've written this code :</p>
  
  <pre><code>    $rootNode = $treeBuilder-&gt;root('media');
  
      $rootNode
          -&gt;children()
              -&gt;arrayNode("medias")
                  -&gt;prototype('array')
                      -&gt;children()
                          -&gt;scalarNode('reference')
                              -&gt;isRequired()
                          -&gt;end()
  
                          -&gt;scalarNode('service')-&gt;end()
  
                          -&gt;arrayNode("identifiers")
                              -&gt;prototype('array')
                                  -&gt;children()
                                      -&gt;scalarNode("label")-&gt;end()
                                      -&gt;scalarNode("key")-&gt;end()
                                  -&gt;end()
                              -&gt;end()
                          -&gt;end()
  
                          -&gt;arrayNode("api")
                              -&gt;prototype('scalar')-&gt;end()
                          -&gt;end()
  
                          -&gt;arrayNode("parameters")
                              -&gt;scalarNode("label")-&gt;end()
  
                              -&gt;arrayNode("page")
                                  -&gt;children()
                                      -&gt;integerNode("factor")-&gt;end()
  
                                      -&gt;arrayNode("styles")
                                          -&gt;prototype('array')
                                              -&gt;children()
                                                  -&gt;scalarNode("css_class")-&gt;end()
                                              -&gt;end()
                                          -&gt;end()
                                      -&gt;end()
  
                                  -&gt;end()
                              -&gt;end()
  
                          -&gt;end()
  
                      -&gt;end()
                  -&gt;end()
              -&gt;end()
          -&gt;end()
      ;
  </code></pre>
  
  <p>But I've the error :</p>
  
  <blockquote>
    <p>Call to undefined method Symfony\Component\Config\Definition\Builder\ArrayNodeDefinition::scalarNode()</p>
  </blockquote>
  
  <p>And I've absolutly no idea where this error come from :/</p>