var select = Ext.create("Terrasoft.EntitySchemaQuery", {rootSchemaName: "InvoiceProduct" });
select.addColumn("Id");
select.addColumn("Invoice");
select.addColumn("TotalAmount");
select.filters.addItem(select.createColumnFilterWithParameter(Terrasoft.ComparisonType.EQUAL, "Invoice",
    this.get("Id")));
select.getEntityCollection(function(response) {
    if (response && response.success) {
        for (var i = 0; i < response.collection.collection.items.length; i++) {
            this.set("MyAmount", this.get("MyAmount") + response.collection.collection.items[i].values.TotalAmount);
        }
    }
}, this);

{
        type: Terrasoft.ViewModelSchemaItem.ATTRIBUTE,
        name: 'Column',
        columnPath: 'Column',
        dataValueType: Terrasoft.DataValueType.FLOAT,
        visible: true
        methodName: 'MyMethod'
}
 
.....;
 
this.methods.MyMethod = function () {
     .....; // здесь код подсчёта суммы
}

var baseInit = this.init;
this.methods.init = function() {
    if (baseInit) {
        baseInit.call();
    }
    this.changeDetailSubscribeByName('product', function() {
        //здесь код подсчёта
    });
};