code: def _delete_existing(self, project_file, session):
          
          # remove existing grid if exists
          existing_elev = session.query(RasterMapFile).\
                                        filter(RasterMapFile.projectFile == project_file).\
                                        filter(RasterMapFile.fileExtension == self.fileExtension).\
                                        all()
          if existing_elev:
              session.delete(existing_elev)
              session.commit()
docs: This will delete existing instances with the same extension