assign calculated grid row value into field

Hello All,

can any one suggest that how can i assign calculated grid row value into field of another object.

let say i have two object  Obj1 and Obj2 .

Obj1 has relationship ( one  Obj1 to many Obj2 ) and field of Obj2 is rating configured in grid now

i calculate sum of all rating using  

<script>

function getSum()

{

var sum=rbf_sumGridColumn2(0, "rating");

}

</script>

and i want value of sum should be assigned into Avg field of Obj1 .

i use rbf_setFieldValue('Avg ' , sum ); but didn't get any result.

thanks.