fix(projects): don't overwrite existing Sales Order project link

Creating a Project with a `sales_order` set used `frappe.db.set_value`
to unconditionally write the Sales Order's `project` field. When several
projects were created for the same Sales Order, each new project silently
overwrote the previous link, leaving earlier projects orphaned.

Only back-link the Sales Order when it is not already tied to another
project, and write the value through the document's `db_set` so the
modified timestamp and realtime update are handled. A warning is shown
when an existing link is left untouched.

Fixes #52179

erpnext/projects/doctype/project/project.py

@@ -240,8 +240,29 @@ class Project(Document):
 
 	def after_insert(self):
 		self.copy_from_template("after_insert")
-		if self.sales_order:
-			frappe.db.set_value("Sales Order", self.sales_order, "project", self.name)
+		self.link_with_sales_order()
+
+	def link_with_sales_order(self) -> None:
+		"""Back-link the source Sales Order to this project.
+
+		The link is set only when the Sales Order is not already tied to another
+		project, so projects created concurrently for the same Sales Order cannot
+		overwrite each other's reference.
+		"""
+		if not self.sales_order:
+			return
+
+		sales_order = frappe.get_doc("Sales Order", self.sales_order)
+		if sales_order.project and sales_order.project != self.name:
+			frappe.msgprint(
+				_("Sales Order {0} is already linked to Project {1}, skipping the link.").format(
+					self.sales_order, sales_order.project
+				),
+				alert=True,
+			)
+			return
+
+		sales_order.db_set("project", self.name)
 
 	def on_trash(self):
 		frappe.db.set_value("Sales Order", {"project": self.name}, "project", "")

erpnext/projects/doctype/project/test_project.py

@@ -174,6 +174,24 @@ class TestProject(ERPNextTestSuite):
 		so.reload()
 		self.assertFalse(so.project)
 
+	def test_sales_order_link_is_not_overwritten_by_second_project(self):
+		so = make_sales_order()
+
+		first_project = make_project_from_so(so.name).save()
+		so.reload()
+		self.assertEqual(so.project, first_project.name)
+
+		# A second project for the same sales order must not steal the link.
+		second_project = frappe.get_doc(
+			doctype="Project",
+			project_name="Second project for same sales order",
+			sales_order=so.name,
+		).insert()
+		self.assertEqual(second_project.sales_order, so.name)
+
+		so.reload()
+		self.assertEqual(so.project, first_project.name)
+
 	def test_project_with_template_tasks_having_common_name(self):
 		# Step - 1: Create Template Parent Tasks
 		template_parent_task1 = create_task(subject="Parent Task - 1", is_template=1, is_group=1)