Calculated field using ESQ

Hi,

I'm trying to add a field - User Rating on the contacts page which is calculated as a sum of a user’s activity duration in minutes, for all activities in the last 60 days. I've added the field, and i understand I must use ESQ to retrieve data and calculate, but I'm unable to find an implementation example in the documentation of BPM Online. 

Could someone possibly suggest? Or atleast provide me a similar implementation which I can use as an example and proceed ahead?

I made an attempt with the ESQ code, but not sure where to place this.

var esq = this.Ext.create(Terrasoft.EntitySchemaQuery, {

rootSchemaName: "Activity"

});

esq.addColumn("DurationInMinutes", "ActivityDuration");

var contact = this.get("Name").value;

var esqFirstFilter = esq.createColumnFilterWithParameter(Terrasoft.ComparisonType.EQUAL,"Activity.Contact", contact);

esq.filters.add("esqFirstFilter", esqFirstFilter);

esq.addAggregationSchemaColumn("DurationInMinutes",Terrasoft.AggregationType.SUM, "ActivitiesDurationSum", Terrasoft.AggregationEvalType.ALL);

esq.getEntity(function(result) {

if (!result.success) {

this.showInformationDialog("Data query error");

return;

}

return (result.entity.get("ActivitiesDurationSum"));

}, this);

Thanks and Regards,
Sushanth.