Add fetch value in a Link field set by default not refreshing

Hi! I have a Doctype call “Viaticos” with a child table call “Itinerario”. I have a field in the child table “fecha” where I want to choose the date and get the exchange rate. I have this script, it works, but I have to touch the link field cambio = Currency Exchange, because it does not refresh automatically. How can I do that?

frappe.ui.form.on("Itinerario", "fecha", function(frm, cdt, cdn) {
    var child = locals[cdt][cdn];
    frappe.model.set_value(child.doctype,, "cambio", child.fecha + "-USD-ARS");
    cur_frm.add_fetch("cambio", "exchange_rate", "cotizacion");

What do you mean by “touch the link field” ? Can you force the field to refresh using refresh_field ?

I am sorry, what I mean is to refresh the link field manually. The script has a refresh field, but still does not work…

The value in link field should be valid existing document.

Check if the value of child.fecha + "-USD-ARS" is a valid document.

If you use set_value, no need to refresh field again.

The value is a valid document…I do not know what could be the problem