code: def by_version(cls, session, package_name, version):
          
          return cls.first(session,
                           join=(Package,),
                           where=((Package.name == package_name),
                                  (cls.version == version)))
docs: Get release 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 instance
          :rtype: :class:`pyshop.models.Release`