title: angular ui-grid grouping from object
body: <p>I am using <a href="http://ui-grid.info/" rel="nofollow noreferrer">ui-grid</a> v3.0.0-rc.22 in my application.</p>
  
  <p>I use grouping in my grid. 
  I have data as follow</p>
  
  <pre><code>[{
              "_id": "559dfaa671c2bd2c0a00002f",
              "title": "dfds",
              "sku": "fdsf",
              "price": "34535",
              "desc": "dsfsdf",
              "main_image": "NOPQR09345.png",
              "added_by": "558a526b977459300a00002b",
              "user": {
                  "_id": "558a526b977459300a00002b",
                  "name": "Merchant",
              }
          }, {
              "_id": "559e0a0f71c2bd2c0a000031",
              "title": "dfdxf",
              "sku": "esdf",
              "price": "345",
              "desc": "xcfgvxdvf",
              "main_image": "NOPQR09345.png",
              "added_by": "558a526b977459300a00002b",
              "user": {
                  "_id": "558a526b977459300a00002b",
                  "name": "Merchant",
              }
          }, {
              "_id": "559e11084a3df01808000029",
              "title": "Product 1",
              "sku": "KJH",
              "price": "12508",
              "desc": "This istest",
              "main_image": "NOPQR09345.png",
              "added_by": "558a6ade977459300a00002c",
              "user": {
                  "_id": "558a6ade977459300a00002c",
                  "name": "Merchant",
              }
          }]
  </code></pre>
  
  <p>I use grouping on user._id and i want to display name in grouping header. For that i use <code>columnDefs</code> as Follow.</p>
  
  <pre><code>    [
         {name: 'Merchant Name', field: 'user._id', grouping: {groupPriority: 0},
                      cellTemplate: '&lt;span style="padding:15px;"&gt;{{row.entity.user.name}}&lt;/span&gt;'},
         {name: 'Name', field: 'title', enableCellEdit: true},
         {name: 'SKU', field: 'sku'},
         {name: 'Category', field: 'pro_category.name'},
         {name: 'Price', field: 'price', treeAggregationType: uiGridGroupingConstants.aggregation.SUM}
  ]
  </code></pre>
  
  <p>The problem is that it show username in grouped column but not showing in groupHeader rows as follow</p>
  
  <p><img src="https://i.stack.imgur.com/vrjtD.png" alt="enter image description here"></p>
  
  <p>How can i do that. Any help greatly appreciated.</p>