Run pre-upgrade checks (optional).

def pre_upgrade(self):
        """Run pre-upgrade checks (optional)."""
        sql = text('select 1 from upgrade where upgrade = :upgrade')
        if not db.engine.execute(
            sql, upgrade='invenio_upgrader_2015_11_12_innodb_removal'
           ).fetchall():
            warnings.warn("Upgrade '{}' was not applied.".format(
                'invenio_upgrader_2015_11_12_innodb_removal'))