a method to retrieve byte data of appdata record

        :param record_key: string with name of record
        :param secret_key: [optional] string used to decrypt data
        :return: byte data for record body

def load(self, record_key, secret_key=''):

        ''' 
            a method to retrieve byte data of appdata record

        :param record_key: string with name of record
        :param secret_key: [optional] string used to decrypt data
        :return: byte data for record body
        '''

        title = '%s.load' % self.__class__.__name__
    
    # validate inputs
        input_fields = {
            'record_key': record_key,
            'secret_key': secret_key
        }
        for key, value in input_fields.items():
            if value:
                object_title = '%s(%s=%s)' % (title, key, str(value))
                self.fields.validate(value, '.%s' % key, object_title)

    # construct path to file
        from os import path
        file_path = path.join(self.collection_folder, record_key)

    # validate existence of file
        if not path.exists(file_path):
            raise Exception('%s(record_key=%s) does not exist.' % (title, record_key))

    # open file and read data
        record_data = open(file_path, 'rb').read()
    
    # decrypt (if necessary)
        if secret_key:
            from labpack.encryption import cryptolab
            record_data = cryptolab.decrypt(record_data, secret_key)
    
        return record_data