fix: read the locked rate from the persisted source row

get_rate_locked_source_row returned the mutable target row, so an unsaved rate or
discount edit on a mapped row was preserved on re-fetch instead of the source
pricing, and the document still failed maintain-same-rate on save. Read the
pricing straight from the linked source row in the database, and cover the
edit-then-refresh case with a test.

erpnext/stock/get_item_details.py

@@ -40,17 +40,28 @@ purchase_doctypes = [
 
 NOT_APPLICABLE_TAX = "N/A"
 
-# Item rows mapped from these source documents must keep the source rate when
-# "maintain same rate" is enabled, so a newer Item Price is never fetched onto them.
+# For each transaction, the child-row link field(s) that point to the source
+# document item, mapped to that source item doctype. When "maintain same rate" is
+# on, a mapped row keeps the persisted source pricing (read straight from that row),
+# so an unsaved edit on the target row can never lock in a non-source rate.
 maintain_same_rate_source_fields = {
-	"Purchase Order": ("supplier_quotation_item",),
-	"Purchase Receipt": ("purchase_order_item",),
-	"Purchase Invoice": ("po_detail", "pr_detail"),
-	"Sales Order": ("quotation_item", "prevdoc_docname"),
-	"Delivery Note": ("so_detail", "si_detail"),
-	"Sales Invoice": ("so_detail", "dn_detail"),
+	"Purchase Order": {"supplier_quotation_item": "Supplier Quotation Item"},
+	"Purchase Receipt": {"purchase_order_item": "Purchase Order Item"},
+	"Purchase Invoice": {"po_detail": "Purchase Order Item", "pr_detail": "Purchase Receipt Item"},
+	"Sales Order": {"quotation_item": "Quotation Item"},
+	"Delivery Note": {"so_detail": "Sales Order Item", "si_detail": "Sales Invoice Item"},
+	"Sales Invoice": {"so_detail": "Sales Order Item", "dn_detail": "Delivery Note Item"},
 }
 
+LOCKED_RATE_FIELDS = [
+	"price_list_rate",
+	"rate",
+	"discount_percentage",
+	"discount_amount",
+	"margin_type",
+	"margin_rate_or_amount",
+]
+
 
 def _preprocess_ctx(ctx):
 	if not ctx.price_list:
@@ -204,11 +215,11 @@ def remove_standard_fields(out: frappe._dict):
 
 
 def get_rate_locked_source_row(ctx: ItemDetailsCtx, doc) -> frappe._dict | None:
-	"""Return the current item row when its rate is locked to a source document.
+	"""Return the persisted source-document row a mapped target row is locked to.
 
-	With "maintain same rate" enabled, a row mapped from a source (e.g. a Purchase
-	Order) can only be saved at the source rate. Fetching a newer Item Price here
-	would set a rate the document can never be saved with, so keep the source rate.
+	The rate is read from the linked source row in the database (not the mutable
+	target row), so a re-fetch always restores the source pricing the maintain-same-
+	rate validator checks against, even after an unsaved edit on the target row.
 	"""
 	if isinstance(doc, str):
 		doc = json.loads(doc)
@@ -218,8 +229,12 @@ def get_rate_locked_source_row(ctx: ItemDetailsCtx, doc) -> frappe._dict | None:
 		return None
 
 	row = next((d for d in doc.get("items") or [] if d.get("name") == ctx.child_docname), None)
-	if row and any(row.get(field) for field in source_fields):
-		return row
+	if not row:
+		return None
+
+	for link_field, source_doctype in source_fields.items():
+		if source_name := row.get(link_field):
+			return frappe.db.get_value(source_doctype, source_name, LOCKED_RATE_FIELDS, as_dict=True)
 	return None
 
 

erpnext/stock/tests/test_get_item_details.py

@@ -315,8 +315,8 @@ class TestGetItemDetail(ERPNextTestSuite):
 			frappe.clear_cache(doctype="Buying Settings")
 
 	def test_maintain_same_rate_keeps_source_discount_on_refetch(self):
-		"""A mapped row with a manual discount has rate != price_list_rate. Re-fetch must
-		keep the source's rate and discount, not just the pre-discount price, or the
+		"""A mapped source row with a discount has rate != price_list_rate. Re-fetch must
+		return the source's rate and discount, not just the pre-discount price, or the
 		recomputed rate diverges from the reference and fails maintain-same-rate on save.
 		"""
 		from frappe.utils import flt
@@ -331,21 +331,22 @@ class TestGetItemDetail(ERPNextTestSuite):
 		frappe.clear_cache(doctype="Buying Settings")
 
 		try:
-			po = create_purchase_order(item_code=item_code, rate=90, qty=1)
+			# source PO carries the discount: list rate 100, 10% off, effective rate 90
+			frappe.flags.dont_fetch_price_list_rate = True
+			po = create_purchase_order(item_code=item_code, qty=1, do_not_save=True)
+			po.buying_price_list = price_list
+			po.items[0].price_list_rate = 100
+			po.items[0].discount_percentage = 10
+			po.items[0].rate = 90
+			po.insert()
+			po.submit()
+			frappe.flags.dont_fetch_price_list_rate = False
 
 			row_name = "pr-row-1"
-			# price_list_rate 100 with a 10% discount gives the effective rate 90.
 			pr_doc = {
 				"doctype": "Purchase Receipt",
 				"items": [
-					{
-						"name": row_name,
-						"item_code": item_code,
-						"purchase_order_item": po.items[0].name,
-						"price_list_rate": 100,
-						"rate": 90,
-						"discount_percentage": 10,
-					}
+					{"name": row_name, "item_code": item_code, "purchase_order_item": po.items[0].name}
 				],
 			}
 			ctx = frappe._dict(
@@ -375,3 +376,38 @@ class TestGetItemDetail(ERPNextTestSuite):
 		finally:
 			frappe.db.set_single_value("Buying Settings", "maintain_same_rate", original)
 			frappe.clear_cache(doctype="Buying Settings")
+			frappe.flags.dont_fetch_price_list_rate = False
+
+	def test_refetch_restores_source_rate_after_target_edit(self):
+		"""Editing a mapped row's rate then re-fetching must restore the persisted source
+		rate (read from the linked row), not lock in the edit, so the document still saves.
+		"""
+		from frappe.utils import flt
+
+		from erpnext.buying.doctype.purchase_order.purchase_order import make_purchase_receipt
+		from erpnext.buying.doctype.purchase_order.test_purchase_order import create_purchase_order
+
+		item = "_Test Item"
+		original = frappe.db.get_single_value("Buying Settings", "maintain_same_rate")
+		original_action = frappe.db.get_single_value("Buying Settings", "maintain_same_rate_action")
+		frappe.db.set_single_value("Buying Settings", "maintain_same_rate", 1)
+		frappe.db.set_single_value("Buying Settings", "maintain_same_rate_action", "Stop")
+		frappe.clear_cache(doctype="Buying Settings")
+
+		try:
+			po = create_purchase_order(item_code=item, qty=1, rate=90)
+			pr = make_purchase_receipt(po.name)
+			pr.insert()
+
+			# user edits the mapped row to a non-source rate
+			pr.items[0].price_list_rate = 200
+			pr.items[0].rate = 200
+
+			# a re-fetch must restore the persisted source (PO) rate, not keep the edit
+			pr.process_item_selection(item_idx=pr.items[0].idx)
+			self.assertEqual(flt(pr.items[0].rate), 90)
+			pr.save()  # must not raise the maintain-same-rate check
+		finally:
+			frappe.db.set_single_value("Buying Settings", "maintain_same_rate", original)
+			frappe.db.set_single_value("Buying Settings", "maintain_same_rate_action", original_action)
+			frappe.clear_cache(doctype="Buying Settings")