Returns the instance's column values as a tab-separated line. A newline is not included.

        - `include_readonly`: if false, returns only fields that can be inserted into database.

def to_tsv(self, include_readonly=True):
        '''
        Returns the instance's column values as a tab-separated line. A newline is not included.

        - `include_readonly`: if false, returns only fields that can be inserted into database.
        '''
        data = self.__dict__
        fields = self.fields(writable=not include_readonly)
        return '\t'.join(field.to_db_string(data[name], quote=False) for name, field in iteritems(fields))