I'm looking for a "best practices" method of handling display updates on a parent form when that data is added on a child form. Aggravating this is that private data sessions are used for all forms. Possibly simplifying it is the use of the Mere Mortals - I looked into its implementation of subjects and observers, and that doesn't feel quite right.

Parent Form - Datasession 2
customers, invoices, lineitems business objects in parent-child relationships, with v_customers v_invoices and v_lineitems cursors all open with buffering to cancel changes.

v_lineitems is shown in a grid on the form, along with an Add LineItem button that opens Child Form.

Child Form - Datasession 3
lineitems business object with v_lineitems cursor open.


Questions:
What's the best place for lineitems.add(), to minimize juggling state and/or adding blank records?
Once the record is added, how do I update the lineitems grid, preferably with the ability to cancel changes?