fix: ensure that additional discount amount is not mapped repeatedly

(cherry picked from commit feb62102d9)

erpnext/controllers/accounts_controller.py

@@ -39,6 +39,8 @@ from erpnext.accounts.doctype.pricing_rule.utils import (
 )
 from erpnext.accounts.general_ledger import get_round_off_account_and_cost_center
 from erpnext.accounts.party import (
+	PURCHASE_TRANSACTION_TYPES,
+	SALES_TRANSACTION_TYPES,
 	get_party_account,
 	get_party_account_currency,
 	get_party_gle_currency,
@@ -2918,6 +2920,83 @@ class AccountsController(TransactionBase):
 			x["transaction_currency"] = self.currency
 			x["transaction_exchange_rate"] = self.get("conversion_rate") or 1
 
+	def after_mapping(self, source_doc):
+		self.set_discount_amount_after_mapping(source_doc)
+
+	def set_discount_amount_after_mapping(self, source_doc):
+		"""
+		Ensures that Additional Discount Amount is not copied repeatedly
+		for multiple mappings of a single source transaction.
+		"""
+
+		# source and target doctypes should both be buying / selling
+		for transaction_types in (PURCHASE_TRANSACTION_TYPES, SALES_TRANSACTION_TYPES):
+			if self.doctype in transaction_types and source_doc.doctype in transaction_types:
+				break
+
+		else:
+			return
+
+		# ensure both doctypes have discount_amount field
+		if not self.meta.get_field("discount_amount") or not source_doc.meta.get_field("discount_amount"):
+			return
+
+		# ensure discount_amount is set in source doc
+		if not source_doc.discount_amount:
+			return
+
+		# ensure additional_discount_percentage is not set in the source doc
+		if source_doc.get("additional_discount_percentage"):
+			return
+
+		item_doctype = self.meta.get_field("items").options
+		item_meta = frappe.get_meta(item_doctype)
+
+		reference_fieldname = next(
+			(
+				row.fieldname
+				for row in item_meta.fields
+				if row.fieldtype == "Link"
+				and row.options == source_doc.doctype
+				and not row.get("is_custom_field")
+			),
+			None,
+		)
+
+		if not reference_fieldname:
+			return
+
+		doctype_table = frappe.qb.DocType(self.doctype)
+		item_table = frappe.qb.DocType(item_doctype)
+		discount_already_applied = (
+			frappe.qb.from_(doctype_table)
+			.where(doctype_table.docstatus == 1)
+			.where(doctype_table.discount_amount != 0)
+			.where(
+				doctype_table.name.isin(
+					frappe.qb.from_(item_table)
+					.select(item_table.parent)
+					.where(item_table[reference_fieldname] == source_doc.name)
+					.distinct()
+				)
+			)
+			.select(Sum(doctype_table.discount_amount))
+		).run()
+
+		if not discount_already_applied:
+			return
+
+		discount_already_applied = flt(discount_already_applied[0][0], self.precision("discount_amount"))
+		if (source_doc.discount_amount * (discount_already_applied - source_doc.discount_amount)) >= 0:
+			# full discount already applied or exceeded
+			self.discount_amount = 0
+		else:
+			self.discount_amount = flt(
+				self.discount_amount - discount_already_applied, self.precision("discount_amount")
+			)
+
+		self.calculate_taxes_and_totals()
+
 
 @frappe.whitelist()
 def get_tax_rate(account_head):