code: def prepareInsert(self, oself, store):
          
          if self.relative:
              fspath = self.__get__(oself)
              oself.__dirty__[self.attrname] = self, self.infilter(fspath, oself, store)
docs: Prepare for insertion into the database by making the dbunderlying
          attribute of the item a relative pathname with respect to the store
          rather than an absolute pathname.