If the invoices for a contact are posted 'on_account' then don't allow this to change

invoice/models.py

@@ -200,6 +200,14 @@ class InvoiceContactManager(models.Manager):
         vat_number=None,
         on_account=None,
     ):
+        """Initialise the invoice details for the contact.
+
+        .. note:: The on account flag (``on_account``) can only be switched on.
+                  It can't be switched off.
+                  If we are posting to an accounts package e.g. Xero, then we
+                  should always post on-account if we ever posted on-account.
+
+        """
         if on_account is None:
             on_account = False
         try:
@@ -211,7 +219,7 @@ class InvoiceContactManager(models.Manager):
             if hourly_rate:
                 x.hourly_rate = hourly_rate
                 update = True
-            if on_account != x.on_account:
+            if on_account is True and x.on_account is False:
                 x.on_account = on_account
                 update = True
             if vat_number:

invoice/tests/test_invoice_contact.py

@@ -137,6 +137,36 @@ def test_init_invoice_contact_hourly_rate():
     assert "" == x.vat_number
 
 
+@pytest.mark.parametrize(
+    "init_on_account,on_account,expect",
+    [
+        (True, True, True),
+        (True, False, True),
+        (False, False, False),
+        (False, True, True),
+    ],
+)
+@pytest.mark.django_db
+def test_init_invoice_on_account(init_on_account, on_account, expect):
+    """Initialise the on-account status for the contact.
+
+    .. note:: The on account flag (``on_account``) can only be switched on.
+              It can't be switched off.
+              If we are posting to an accounts package e.g. Xero, then we
+              should always post on-account if we ever posted on-account.
+
+    """
+    contact = ContactFactory()
+    InvoiceContact.objects.init_invoice_contact(
+        contact, on_account=init_on_account,
+    )
+    x = InvoiceContact.objects.init_invoice_contact(
+        contact, on_account=on_account,
+    )
+    x.refresh_from_db()
+    assert expect == x.on_account
+
+
 @pytest.mark.django_db
 def test_init_invoice_contact_vat_number():
     contact = ContactFactory()