'Return a string based \'column specification\' for this
:class:`.ForeignKey`.
This is usually the equivalent of the string-based "tablename.colname"
argument first passed to the object\'s constructor.'

def _get_colspec(self, schema=None):

    if schema:
        (_schema, tname, colname) = self._column_tokens
        return ('%s.%s.%s' % (schema, tname, colname))
    elif (self._table_column is not None):
        return ('%s.%s' % (self._table_column.table.fullname, self._table_column.key))
    else:
        return self._colspec
