Thanks for your response. The problem is my datasource entity does not have a DonorID...
It holds a navigationproperty Donors. This poroperty does have an DonorsId. To clarify:
edsDoneationSchedule contains a collection of donor_DonationSchedule entities. These donor_DonationSchedule entities conform to the scheduler required fields like start/end/subject/etc.. Creating/Editing these 'appointment entities' works fine. There is a working example with an entitydatasource in the demo-section of this site.
Now i like to add a custom 'field' a donor_Donors. So in the database this is 'just' a Guid (DonorsId), however, to the EF it is a navigation property, hence donor_DonationSchedule does not contain a DonorsId!
I can't seem to 'intercept' the insert/update of the edsDoneationSchedule :( At that point I would set the correct reference to donor_DonationSchedule.Donors with the combobox selected value.
Maybe I should 'revert' to an sqldatasource, but this would 'break' the architecture since we use EF...