You can achieve complex data binding in the treegrid by using the dot(.) operator in the column.field.
var treeGridObj = new ej.treegrid.TreeGrid({
dataSource: complexData,
childMapping: 'subtasks',
height: 315,
treeColumnIndex: 1,
columns: [
{ field: 'taskID', headerText: 'Task ID', width: 90, textAlign: 'Right' },
{ field: 'taskName', headerText: 'Task Name', width: 180 },
{ field: 'assignee.firstName', headerText: 'Assignee', width: 90},
{ field: 'duration', headerText: 'Duration', width: 80, textAlign: 'Right' }
]
});
treeGridObj.appendTo('#TreeGrid');