title: How to get value of valueField of combobox in ExtJs
body: <p>I create a combobox in Extjs where i am showing name of Reports and when user select any report, at back end id of report should be posted. I am not able to get id of report. I am using store to fill data in combobox.</p>
  
  <p>I create a model in Extjs like </p>
  
  <pre><code>Ext.define('Report', {
      extend: 'Ext.data.Model',
      fields: [
          {name: 'ReportId', type: 'int'},
          {name: 'DisplayName',  type: 'string'}
      ]
  });
  </code></pre>
  
  <p>Store is like </p>
  
  <pre><code> var reportStore = Ext.create('Ext.data.Store', {
      model: 'Report',
      autoLoad: true,
      proxy: {
          type: 'ajax',
          url: 'reportlist.json'
  });
  </code></pre>
  
  <p>comboBox like</p>
  
  <pre><code>var comboReports = Ext.create('Ext.form.field.ComboBox', {
      displayField: 'DisplayName',
     valueField: 'ReportId',
     fieldLabel: 'Report',
      store: reportStore,
      queryMode: 'local',
      emptyText: 'Select a report',
      selectOnFocus: true,
      listeners: {
              select: function(combo, selection) {
                  if (combo.getValue()) {
          //showData(reportId); here i want the id of seleted report to passed.
  
                  }
              }
          }
      });
  </code></pre>