Add only modified records of xxx2Many
The xxx2Many field on client side add always all the known records, no matter if they are already added or not.
The idea was to be sure that what the user see is what is saved. But this forces the server to check if all records are already added or not. This can be slow on very large list.
So the idea is to only add the modified records and of course to mark the record as modified when it is added.