feat: add patch for dunning

erpnext/patches.txt

@@ -303,3 +303,4 @@ erpnext.patches.v13_0.modify_invalid_gain_loss_gl_entries
 erpnext.patches.v13_0.fix_additional_cost_in_mfg_stock_entry
 erpnext.patches.v13_0.set_status_in_maintenance_schedule_table
 erpnext.patches.v13_0.add_default_interview_notification_templates
+erpnext.patches.v13_0.single_to_multi_dunning

erpnext/patches/v13_0/single_to_multi_dunning.py

@@ -0,0 +1,46 @@
+import frappe
+from erpnext.accounts.general_ledger import make_reverse_gl_entries
+
+def execute():
+	frappe.reload_doc("accounts", "doctype", "overdue_payment")
+	frappe.reload_doc("accounts", "doctype", "dunning")
+
+	all_dunnings = frappe.get_all("Dunning", pluck="name")
+	for dunning_name in all_dunnings:
+		dunning = frappe.get_doc("Dunning", dunning_name)
+		if not dunning.sales_invoice:
+			# nothing we can do
+			continue
+
+		if dunning.overdue_payments:
+			# something's already here, doesn't need patching
+			continue
+
+		payment_schedules = frappe.get_all("Payment Schedule",
+			filters={"parent": dunning.sales_invoice},
+			fields=[
+				"parent as sales_invoice",
+				"name as payment_schedule",
+				"payment_term",
+				"due_date",
+				"invoice_portion",
+				"payment_amount",
+				# at the time of creating this dunning, the full amount was outstanding
+				"payment_amount as outstanding",
+				"'0' as paid_amount",
+				"discounted_amount"
+			]
+		)
+
+		dunning.extend("overdue_payments", payment_schedules)
+		dunning.validate()
+
+		dunning.flags.ignore_validate_update_after_submit = True
+		dunning.save()
+
+		if dunning.status != "Resolved":
+			# With the new logic, dunning amount gets recorded as additional income 
+			# at time of payment. We don't want to record the dunning amount twice,
+			# so we reverse previous GL Entries that recorded the dunning amount at
+			# time of submission of the Dunning.
+			make_reverse_gl_entries(voucher_type="Dunning", voucher_no=dunning.name)