I currently have this code, it works well, except if I am creating a new record set value. 

On that time it raises an exception and than the constraint message.

Is they any way to achieve the goal, with either of the two methods, or sql constraint ?

class ApiPartnerProduct(models.Model):
    _name = 'api_partner_product'
    _description = "API Product to Partner relationship for Electronic API"
    _sql_constraints = [('vendorcode_partnerid_unique', 'UNIQUE (x_partner_id & x_vendor_code)', 'API Vendor & Vendor Product Code combination is duplicated.')]

    x_product_tmpl_id = fields.Many2one(comodel_name='product.template', string="Product Code", required="1")
    x_partner_id = fields.Many2one(comodel_name='api_connector_core', string="API Vendor", ondelete='cascade',
                                   required="1")
    x_vendor_code = fields.Char(string="Vendor Product Code", required="1")
    x_last_update = fields.Date(string="Last API Update")

    @api.onchange('x_vendor_code')
    def _check_vendor_partner_duplicate(self):
        history=self._origin.read(['x_vendor_code'])
        ids = self.env['api_partner_product'].search(
            [('x_vendor_code', '=', self.x_vendor_code), ('x_partner_id', '=', self.x_partner_id.id)])
        self.res={}
        if len(ids) >= 1:
            if len(history) != 0:
                self.x_vendor_code=history[0]['x_vendor_code']
                self.x_partner_id = history[0]['x_parner_id']
            else :
                self.x_vendor_code=None
                return
            self.res = {'warning': {'title': "Duplicated Records",'message': "API Vendor & Vendor Product Code combination is duplicated."}}
        return self.res