code: def process_form(self, instance, field, form, empty_marker=None,
                       emptyReturnsMarker=False):
          
          fieldName = field.getName()
          if fieldName + "_uid" in form:
              uid = form.get(fieldName + "_uid", '')
              if field.multiValued and\
                      (isinstance(uid, str) or isinstance(uid, unicode)):
                  uid = uid.split(",")
          elif fieldName in form:
              uid = form.get(fieldName, '')
              if field.multiValued and\
                      (isinstance(uid, str) or isinstance(uid, unicode)):
                  uid = uid.split(",")
          else:
              uid = None
          return uid, {}
docs: Return a UID so that ReferenceField understands.