title: ExtJs 4 MultiSelect Edit Form doesn't load selections
body: <p>I need some help about combo box with multiple select in Extjs 4.</p>
  
  <p>I need it in my forms for a simple ManyToMany relation splitted by a central table:</p>
  
  <p><strong>User -&lt; User_Group >- Group.</strong></p>
  
  <p>Where:</p>
  
  <p><strong>User</strong>: id, name, year</p>
  
  <p><strong>User_Group</strong>: user_id, group_id</p>
  
  <p><strong>Group</strong>: id, name</p>
  
  <p>I have no problem in the creation form of a User and i can save to databse by php all the groups I've associated by the combobox.</p>
  
  <p>Now, I have a grid with all my users, and when I try to start edit one of them, the selections of the combobox are not loaded, but the combobox's field show the ids of the corrected selections.</p>
  
  <p>For example:</p>
  
  <p>When I try edit this user (json code)</p>
  
  <pre><code>{
      "id": 86,
      "name": "tempname",
      "year": 1492,
      "groups_id": [1,2,3]
  }
  </code></pre>
  
  <p>The Edit form is filled with all the information and the field of combobox shows <strong>1,2,3</strong> , but <strong>nothing is selected</strong> in its dropdown menu.</p>
  
  <p>This is my combobox:</p>
  
  <pre><code>{
      xtype: 'combobox',
      multiSelect: true,
  
      name: 'groups_id',
      fieldLabel: 'Group/s',
  
      valueField: 'id',
      displayField: 'name',
  
      store: 'Groups',
      allowBlank: false
  }
  </code></pre>
  
  <p>So, why nothing is getting selected on the load of thtat form? And why the ids I pass to load the combobox (groups_id) are not getting really bind to it? 
  It's wrong the json structure of a user?</p>
  
  <p>I've passed two days on this...and yes...I'm pretty new to ExtJs ;)</p>