jQuery Grid Plugin - jqGrid

": "1",
"total": 1,
"records": "6",
"rows": [
    {
        "head": {
            "student_name": "Mr S. Jack ",
            "year": 2007

        },
        "sub": [
            {
                "course_description": "Math ",
                "date": "22-04-2010",
                "number": 1,
                "time_of_add": "2:00",
                "day": "today"
            }
        ]

      }
]
}

my jqgrid code is as follows

jQuery("#"+subgrid_table_id).jqGrid({
url:"http://localhost/stud/beta/web/GetStud.php?sid="+sid,
dtatype: "json",
colNames: ['Stud Name','Year','Date'.'Number'],
colModel: [ {name:'Stud Name',index:'student_name', width:100, jsonmap:"student_name"},
{name:'Year',index:'year', width:100, jsonmap:"year"},
{name:'Date',index:'date', width:100, jsonmap:"date"},
{name:'Number',index:'number', width:100, jsonmap:"number"}
],
height:'100%',
jsonReader: { repeatitems : false, root:"head" },
});

So now the problem is as my data i.e. student_name and year is under "head" , the jqgrid is enable to locate these two fields. at the same time other two column values i.e. Date and Number lies under "sub" and even those columns i am not be able to map it with jqgrid

so kindly help me how to located these attributes in JQGrid.

Thanks