Sort advance payment invoices on Function fields of sale

Closes #15131

modules/sale_advance_payment/sale.py

@@ -8,7 +8,7 @@ from simpleeval import simple_eval
 from trytond import backend, config
 from trytond.i18n import gettext
 from trytond.model import (
-    DeactivableMixin, ModelSQL, ModelView, Workflow, fields)
+    DeactivableMixin, ModelSQL, ModelView, Workflow, fields, sort)
 from trytond.modules.company.model import (
     CompanyMultiValueMixin, CompanyValueMixin)
 from trytond.modules.currency.fields import Monetary
@@ -346,12 +346,16 @@ class Sale(metaclass=PoolMeta):
             self.advance_payment_lines = advance_payment_term.get_lines(self)
 
     def get_advance_payment_invoices(self, name):
+        pool = Pool()
+        Invoice = pool.get('account.invoice')
+        field = getattr(self.__class__, name)
+        order = field.order if field.order is not None else Invoice._order
         invoices = set()
         for line in self.advance_payment_lines:
             for invoice_line in line.invoice_lines:
                 if invoice_line.invoice:
-                    invoices.add(invoice_line.invoice.id)
-        return list(invoices)
+                    invoices.add(invoice_line.invoice)
+        return sort(invoices, order)
 
     @classmethod
     def search_advance_payment_invoices(cls, name, clause):

modules/sale_advance_payment/tests/scenario_advance_payment.rst

@@ -392,7 +392,7 @@ it::
     ...     handle_exception.form.recreate_invoices.find())
     >>> handle_exception.execute('handle')
     >>> sale.reload()
-    >>> _, inv_recreated = sale.advance_payment_invoices
+    >>> inv_recreated, _ = sale.advance_payment_invoices
     >>> inv_recreated.total_amount
     Decimal('10.00')
     >>> assertEqual(inv_recreated.invoice_date, next_week)