title: dynamically set fields to a extjs data store
body: <p>I am trying to dynamically set fields to a extjs data store so that I could dynamically create a different grid at run time.</p>
  
  <p><strong>Case A</strong> works for me. But when I use as in <strong>Case B</strong>, the store's proxy hangs on to the previous model and so the grid rendering is messed up.</p>
  
  <p>What is the really the difference between these two?</p>
  
  <p><strong>Case A</strong></p>
  
  <pre><code>Ext.define('FDG.store.reading.FDGDynamicGridStore', {
      extend: 'Ext.data.Store'
  });
  
  var fdgstore = Ext.create('FDG.store.reading.FDGDynamicGridStore', {
          fields: fields,
          proxy: {
              type: 'memory',
              reader: {
              type: 'json',
              totalProperty: 'tc',
              root: 'Result'
              }
          }
          });
  fdgstore.loadRawData(output);
  this.reconfigure(fdgstore, columns);
  </code></pre>
  
  <p><strong>Case B</strong></p>
  
  <pre><code>Ext.define('FDG.store.reading.FDGDynamicGridStore', {
      extend: 'Ext.data.Store',    
      proxy: {
          type: 'memory',
          reader: {
              type: 'json',
              totalProperty: 'tc',
              root: 'Result'
          }
      }
  });
  
  var fdgstore = Ext.create('FDG.store.reading.FDGDynamicGridStore', {
          fields: fields
          });
  fdgstore.loadRawData(output);
  this.reconfigure(fdgstore, columns);
  </code></pre>