title: How to calculate total of one field in the store
body: <p>I have a grid within a form as in the attached image.  When the the customer name is changed, then the grid store is loaded with records corresponding to the customer.  I want the balance textfield to be populated with the sum of Amount due column.</p>
  
  <p>The image is  <a href="http://www.sencha.com/forum/showthread.php?210737-Grid-in-a-form&amp;p=815471" rel="nofollow">here</a>.</p>