def _str_foreign_keys(self):
        keys = []
        for col in self.foreign_keys:
            keys.append("%s.%s" % (col.table, col.name))
        if self.keys_per_column is not None and len(keys) > self.keys_per_column:
            keys = keys[0:self.keys_per_column] + ['(+ {0} more)'.format(len(keys) - self.keys_per_column)]
        return ", ".join(keys)