Odoo 10 Query sql does not return value

I have a simple sql query but it does not return any value to me. Where can the error be? Thanks

@api.model_cr
def _compute_impuestos(self):
    """ Event Question main report """
    tools.drop_view_if_exists(self._cr, 'product.template')
    self._cr.execute(""" 
        SELECT
            id
        FROM
            product_template
        LIMIT 1
    """)
    res = self._cr.fetchone()
    return res