perf: Fetching exchange rate on every line item slows down PO (#25345)

* fix: Dont fetch exchange rates for each line item once fetched at parent
`

* perf: Use price list conversion rate from parent

- If price list conversion rate exists in args already from earlier call, use that
- `get_price_list_currency_and_exchange_rate` wont be called for each child row

Co-authored-by: Nabin Hait <nabinhait@gmail.com>

erpnext/public/js/controllers/transaction.js

@@ -1103,6 +1103,8 @@ erpnext.TransactionController = erpnext.taxes_and_totals.extend({
 				to_currency: to_currency,
 				args: args
 			},
+			freeze: true,
+			freeze_message: __("Fetching exchange rates ..."),
 			callback: function(r) {
 				callback(flt(r.message));
 			}

erpnext/stock/get_item_details.py

@@ -609,8 +609,12 @@ def get_price_list_rate(args, item_doc, out):
 	meta = frappe.get_meta(args.parenttype or args.doctype)
 
 	if meta.get_field("currency") or args.get('currency'):
-		pl_details = get_price_list_currency_and_exchange_rate(args)
-		args.update(pl_details)
+		if not args.get("price_list_currency") or not args.get("plc_conversion_rate"):
+			# if currency and plc_conversion_rate exist then
+			# `get_price_list_currency_and_exchange_rate` has already been called
+			pl_details = get_price_list_currency_and_exchange_rate(args)
+			args.update(pl_details)
+
 		if meta.get_field("currency"):
 			validate_conversion_rate(args, meta)
 
@@ -1000,6 +1004,8 @@ def apply_price_list(args, as_doc=False):
 	args = process_args(args)
 
 	parent = get_price_list_currency_and_exchange_rate(args)
+	args.update(parent)
+
 	children = []
 
 	if "items" in args:
@@ -1064,7 +1070,7 @@ def get_price_list_currency_and_exchange_rate(args):
 	return frappe._dict({
 		"price_list_currency": price_list_currency,
 		"price_list_uom_dependant": price_list_uom_dependant,
-		"plc_conversion_rate": plc_conversion_rate
+		"plc_conversion_rate": plc_conversion_rate or 1
 	})
 
 @frappe.whitelist()