fix(patch): updates maintenance schedule field for old docs
diff --git a/erpnext/patches.txt b/erpnext/patches.txt
index ee9060b..937465b 100644
--- a/erpnext/patches.txt
+++ b/erpnext/patches.txt
@@ -313,4 +313,5 @@
 erpnext.patches.v13_0.create_pan_field_for_india #2
 erpnext.patches.v14_0.delete_hub_doctypes
 erpnext.patches.v13_0.create_ksa_vat_custom_fields
+erpnext.patches.v13_0.update_maintenance_schedule_field_in_visit
 erpnext.patches.v14_0.migrate_crm_settings
diff --git a/erpnext/patches/v13_0/update_maintenance_schedule_field_in_visit.py b/erpnext/patches/v13_0/update_maintenance_schedule_field_in_visit.py
new file mode 100644
index 0000000..450c00e
--- /dev/null
+++ b/erpnext/patches/v13_0/update_maintenance_schedule_field_in_visit.py
@@ -0,0 +1,22 @@
+
+import frappe
+
+
+def execute():
+	# Updates the Maintenance Schedule link to fetch serial nos
+	from frappe.query_builder.functions import Coalesce
+	mvp = frappe.qb.DocType('Maintenance Visit Purpose')
+	mv = frappe.qb.DocType('Maintenance Visit')
+
+	frappe.qb.update(
+		mv
+	).join(
+		mvp
+	).on(mvp.parent == mv.name).set(
+		mv.maintenance_schedule,
+		Coalesce(mvp.prevdoc_docname, '')
+	).where(
+		(mv.maintenance_type == "Scheduled")
+		& (mvp.prevdoc_docname.notnull())
+		& (mv.docstatus < 2)
+	).run(as_dict=1)