Rollbase Portal

I am connected to sports2000 database as external database and created the object for customer table and for same object i have object edit portal page. In that page customer id is read only field and it is a primary key , when i try to edit the record i get error message "DataObject with id 51 not found" and takes me to the customer detail page with customer id 51  and if i remove the read only attribute from customer id then i get the error message saying "Customer Id is already in use". Why am i getting this error?