models.py 文件源码

def instrument_code(cls):  # @NoSelf
        '''returns the sql expression returning the second letter of the channel field,
        or NULL if the latter has not length 3'''
        # return an sql expression matching the last char or None if not three letter channel
        return substr(cls.channel, 2, 1)
        # return case([(func.length(cls.channel) == 3, func.substr(cls.channel, 2, 1))], else_=null())