code: def by_filename(cls, session, release, filename):
          
          return cls.first(session,
                           where=(ReleaseFile.release_id == release.id,
                                  ReleaseFile.filename == filename,
                                  ))
docs: Get a release file for a given release and a given filename.
  
          :param session: SQLAlchemy session
          :type session: :class:`sqlalchemy.Session`
  
          :param release: release
          :type release: :class:`pyshop.models.Release`
  
          :param filename: filename of the release file
          :type filename: unicode
  
          :return: release file
          :rtype: :class:`pyshop.models.ReleaseFile`