title: Save nested form data back to server in ExtJS
body: <p>I have a Model that contains an association to another Model. I am able to display the nested data into a form by using the mapping attribute on the field. Example:</p>
  
  <pre><code>Ext.define('Example.model.Request', {
      extend: 'Ext.data.Model',
  
      fields: [
          {
              name: 'id',
              type: Ext.data.Types.NUMBER,
              useNull: false
          }
          {
              name: 'plan_surveyor',
              mapping: 'plan.surveyor',
              type: Ext.data.Types.STRING
          }
      ],
  
      associations: [
                 {type: 'hasOne', associationKey: 'plan', getterName:'getPlan', model: 'Specs.model.Plan'}
                ],
  
      proxy: {
              type: 'direct',
              api: {
                  read: requestController.load,
                   update: requestController.update,
              },
              reader: {
                  type: 'json',
                  root: 'records'
              },
              writer: {
                  type: 'json',
                  writeAllFields: true,
                  nameProperty: 'mapping'
              }
          }
  
      });
  </code></pre>
  
  <p>Using this method, I can display the plan.surveyor value in the form by reference plan_surveyor. I call Form.loadRecord(model) to pull the data from the model into the form.</p>
  
  <p>However, now that I'm trying to send the data back to the server, I get the error:</p>
  
  <p><code>Error performing action. Please report the following: "Unrecognized field "plan.surveyor"</code></p>
  
  <p>I am attempting to save to the server by first calling Form.updateRecord(model), then model.save(). Is there a way to have the Writer understand that 'plan.surveyor' is not a property name but instead to properly handle nesting?</p>
  
  <p>Am I doing this the right way to start with, or should I just be handling the setting of the form data and loading back into the model in a more manual fashion? It seems that nested data is not all that well supported in general - any recommendations?</p>