如何让ean13在odoo8中独一无二

class MyModel(models.Model):
    _name = 'my.model'

    ean13 = fields.Char(string='EAN13', size=13)

    _sql_constraints = [
        ('ean13_unique', 'unique(ean13)', 'EAN13 must be unique!')
    ]

class MyModel(models.Model):
    _name = 'my.model'

    ean13 = fields.Char(string='EAN13', size=13)

    @api.model
    def create(self, vals):
        if 'ean13' in vals:
            existing_record = self.search([('ean13', '=', vals['ean13'])])
            if existing_record:
                raise ValidationError('EAN13 must be unique!')
        return super(MyModel, self).create(vals)

    @api.multi
    def write(self, vals):
        if 'ean13' in vals:
            existing_record = self.search([('ean13', '=', vals['ean13']), ('id', '!=', self.id)])
            if existing_record:
                raise ValidationError('EAN13 must be unique!')
        return super(MyModel, self).write(vals)