fix: dont update RM items table if not required (frappe#31408)

Currently on PO update RM item table is auto computed again and again,
if there was any transfer/consumption against that then it will be lost.

This change:
1. Disables updating RM table if no change in qty of FG was made. Since
   RM table can't possibly be different with same FG qty.
2. Blocks update completely if qty is changed and RM items are already
   transferred.

erpnext/buying/doctype/purchase_order/test_purchase_order.py

@@ -140,6 +140,43 @@ def test_update_remove_child_linked_to_mr(self):
 		# ordered qty decreases as ordered qty is 0 (deleted row)
 		self.assertEqual(get_ordered_qty(), existing_ordered_qty - 10)  # 0
 
+	def test_supplied_items_validations_on_po_update_after_submit(self):
+		po = create_purchase_order(item_code="_Test FG Item", is_subcontracted=1, qty=5, rate=100)
+		item = po.items[0]
+
+		original_supplied_items = {po.name: po.required_qty for po in po.supplied_items}
+
+		# Just update rate
+		trans_item = [
+			{
+				"item_code": "_Test FG Item",
+				"rate": 20,
+				"qty": 5,
+				"conversion_factor": 1.0,
+				"docname": item.name,
+			}
+		]
+		update_child_qty_rate("Purchase Order", json.dumps(trans_item), po.name)
+		po.reload()
+
+		new_supplied_items = {po.name: po.required_qty for po in po.supplied_items}
+		self.assertEqual(set(original_supplied_items.keys()), set(new_supplied_items.keys()))
+
+		# Update qty to 2x
+		trans_item[0]["qty"] *= 2
+		update_child_qty_rate("Purchase Order", json.dumps(trans_item), po.name)
+		po.reload()
+
+		new_supplied_items = {po.name: po.required_qty for po in po.supplied_items}
+		self.assertEqual(2 * sum(original_supplied_items.values()), sum(new_supplied_items.values()))
+
+		# Set transfer qty and attempt to update qty, shouldn't be allowed
+		po.supplied_items[0].supplied_qty = 2
+		po.supplied_items[0].db_update()
+		trans_item[0]["qty"] *= 2
+		with self.assertRaises(frappe.ValidationError):
+			update_child_qty_rate("Purchase Order", json.dumps(trans_item), po.name)
+
 	def test_update_child(self):
 		mr = make_material_request(qty=10)
 		po = make_purchase_order(mr.name)

erpnext/controllers/accounts_controller.py

@@ -2440,7 +2440,7 @@ def update_bin_on_delete(row, doctype):
 		update_bin_qty(row.item_code, row.warehouse, qty_dict)
 
 
-def validate_and_delete_children(parent, data):
+def validate_and_delete_children(parent, data) -> bool:
 	deleted_children = []
 	updated_item_names = [d.get("docname") for d in data]
 	for item in parent.items:
@@ -2459,6 +2459,8 @@ def validate_and_delete_children(parent, data):
 	for d in deleted_children:
 		update_bin_on_delete(d, parent.doctype)
 
+	return bool(deleted_children)
+
 
 @frappe.whitelist()
 def update_child_qty_rate(parent_doctype, trans_items, parent_doctype_name, child_docname="items"):
@@ -2522,13 +2524,38 @@ def validate_quantity(child_item, new_data):
 		):
 			frappe.throw(_("Cannot set quantity less than received quantity"))
 
+	def should_update_supplied_items(doc) -> bool:
+		"""Subcontracted PO can allow following changes *after submit*:
+
+		1. Change rate of subcontracting - regardless of other changes.
+		2. Change qty and/or add new items and/or remove items
+		        Exception: Transfer/Consumption is already made, qty change not allowed.
+		"""
+
+		supplied_items_processed = any(
+			item.supplied_qty or item.consumed_qty or item.returned_qty for item in doc.supplied_items
+		)
+
+		update_supplied_items = (
+			any_qty_changed or items_added_or_removed or any_conversion_factor_changed
+		)
+		if update_supplied_items and supplied_items_processed:
+			frappe.throw(_("Item qty can not be updated as raw materials are already processed."))
+
+		return update_supplied_items
+
 	data = json.loads(trans_items)
 
+	any_qty_changed = False  # updated to true if any item's qty changes
+	items_added_or_removed = False  # updated to true if any new item is added or removed
+	any_conversion_factor_changed = False
+
 	sales_doctypes = ["Sales Order", "Sales Invoice", "Delivery Note", "Quotation"]
 	parent = frappe.get_doc(parent_doctype, parent_doctype_name)
 
 	check_doc_permissions(parent, "write")
-	validate_and_delete_children(parent, data)
+	_removed_items = validate_and_delete_children(parent, data)
+	items_added_or_removed |= _removed_items
 
 	for d in data:
 		new_child_flag = False
@@ -2539,6 +2566,7 @@ def validate_quantity(child_item, new_data):
 
 		if not d.get("docname"):
 			new_child_flag = True
+			items_added_or_removed = True
 			check_doc_permissions(parent, "create")
 			child_item = get_new_child_item(d)
 		else:
@@ -2561,6 +2589,7 @@ def validate_quantity(child_item, new_data):
 			qty_unchanged = prev_qty == new_qty
 			uom_unchanged = prev_uom == new_uom
 			conversion_factor_unchanged = prev_con_fac == new_con_fac
+			any_conversion_factor_changed |= not conversion_factor_unchanged
 			date_unchanged = (
 				prev_date == getdate(new_date) if prev_date and new_date else False
 			)  # in case of delivery note etc
@@ -2574,6 +2603,8 @@ def validate_quantity(child_item, new_data):
 				continue
 
 		validate_quantity(child_item, d)
+		if flt(child_item.get("qty")) != flt(d.get("qty")):
+			any_qty_changed = True
 
 		child_item.qty = flt(d.get("qty"))
 		rate_precision = child_item.precision("rate") or 2
@@ -2679,8 +2710,9 @@ def validate_quantity(child_item, new_data):
 		parent.update_ordered_and_reserved_qty()
 		parent.update_receiving_percentage()
 		if parent.is_subcontracted:
-			parent.update_reserved_qty_for_subcontract()
-			parent.create_raw_materials_supplied("supplied_items")
+			if should_update_supplied_items(parent):
+				parent.update_reserved_qty_for_subcontract()
+				parent.create_raw_materials_supplied("supplied_items")
 			parent.save()
 	else:  # Sales Order
 		parent.validate_warehouse()