title: Store with custom sorting in Sencha Touch
body: <p>I have a store + model which is connected to a 3rd party plugin (Ext.ux.TouchGridPanel). The plugin calls the store's sort() method properly with the relevant mapping. Everything is working fine, and the store sorts itself. However, I would prefer to add customer sorting to the store. I have tried adding a sortType field into my model:</p>
  
  <pre><code>Ext.regModel("Transactions", {
      fields: [
          {
              name: 'unit',
              type: 'string',
              sortType: function(value) {
                  console.log('PRINT GDAMNIT');
                  return 0;
              }
          },
  
          ...
  
      ]
  });
  </code></pre>
  
  <p>This, however, is not working, and the sortType is not getting called.</p>
  
  <p><strong>TLDR: How to make custom sorting work for stores?</strong></p>