code: def by_release(cls, session, package_name, version):
          
          return cls.find(session,
                          join=(Release, Package),
                          where=(Package.name == package_name,
                                 Release.version == version,
                                 ))
docs: Get release files for a given package
          name and for a given version.
  
          :param session: SQLAlchemy session
          :type session: :class:`sqlalchemy.Session`
  
          :param package_name: package name
          :type package_name: unicode
  
          :param version: version
          :type version: unicode
  
          :return: release files
          :rtype: generator of :class:`pyshop.models.ReleaseFile`