In Construct Preview, OnSave (or OnValidate) events run when you save records in the following cases:
After inserting a record
After editing a record
Through the Event Validation dialog box, by clicking the last event’s status in the Status column
Currently, in Construct Preview, the OnSave event runs for each record in the dataset displayed in the grid or form.
There are use cases where a child record affects a parent record, while still utilizing data from the child row for that manipulation. That operation should only affect one record (the parent) by filtering the affecting SQL with column values passed through the navigation between pages.