fix(grants): prefetch vouchers in admin; idempotent grant voucher email

- create_grant_vouchers: load conference vouchers for selected users in one
  query; update map after creating new vouchers (avoids N+1).
- send_grant_voucher_email: no-op when voucher_email_sent_at is set.
- Clear voucher_email_sent_at when upgrading CO_SPEAKER to GRANT so the
  grant template still sends after a prior voucher email.
- Tests: skip-when-sent; co-speaker upgrade with prior sent_at.

Author: estyxx

backend/grants/admin.py

@@ -301,7 +301,19 @@ def send_reply_email_waiting_list_update(modeladmin, request, queryset):
 @validate_single_conference_selection
 @transaction.atomic
 def create_grant_vouchers(modeladmin, request, queryset):
-    for grant in queryset.order_by("id").select_related("user", "conference"):
+    grants_ordered = list(queryset.order_by("id").select_related("user", "conference"))
+    conference_id = grants_ordered[0].conference_id if grants_ordered else None
+    user_ids = {g.user_id for g in grants_ordered if g.user_id}
+
+    existing_by_user: Dict[int, ConferenceVoucher] = {}
+    if conference_id is not None and user_ids:
+        for voucher in ConferenceVoucher.objects.filter(
+            conference_id=conference_id,
+            user_id__in=user_ids,
+        ):
+            existing_by_user[voucher.user_id] = voucher
+
+    for grant in grants_ordered:
         if grant.status != Grant.Status.confirmed:
             messages.error(
                 request,
@@ -317,18 +329,15 @@ def create_grant_vouchers(modeladmin, request, queryset):
             )
             continue
 
-        existing = (
-            ConferenceVoucher.objects.for_conference(grant.conference)
-            .for_user(grant.user)
-            .first()
-        )
+        existing = existing_by_user.get(grant.user_id)
 
         if not existing:
-            create_conference_voucher(
+            new_voucher = create_conference_voucher(
                 conference=grant.conference,
                 user=grant.user,
                 voucher_type=ConferenceVoucher.VoucherType.GRANT,
             )
+            existing_by_user[grant.user_id] = new_voucher
             create_addition_admin_log_entry(
                 request.user,
                 grant,
@@ -359,7 +368,8 @@ def create_grant_vouchers(modeladmin, request, queryset):
                 change_message="Updated existing Co-Speaker voucher to grant.",
             )
             existing.voucher_type = ConferenceVoucher.VoucherType.GRANT
-            existing.save(update_fields=["voucher_type"])
+            existing.voucher_email_sent_at = None
+            existing.save(update_fields=["voucher_type", "voucher_email_sent_at"])
 
     messages.success(request, "Vouchers created!")
 

backend/grants/tasks.py

@@ -37,6 +37,14 @@ def send_grant_voucher_email(*, grant_id: int) -> None:
     if not conference_voucher:
         return
 
+    if conference_voucher.voucher_email_sent_at is not None:
+        logger.info(
+            "Grant voucher email already sent for user %s conference %s, skipping",
+            grant.user_id,
+            grant.conference_id,
+        )
+        return
+
     visa_page_link = urljoin(settings.FRONTEND_URL, "/visa")
     conference_name = grant.conference.name.localize("en")
 
@@ -85,7 +93,10 @@ def create_and_send_voucher_to_grantee(*, grant_id: int) -> None:
             return
         if conference_voucher.voucher_type == ConferenceVoucher.VoucherType.CO_SPEAKER:
             conference_voucher.voucher_type = ConferenceVoucher.VoucherType.GRANT
-            conference_voucher.save(update_fields=["voucher_type"])
+            conference_voucher.voucher_email_sent_at = None
+            conference_voucher.save(
+                update_fields=["voucher_type", "voucher_email_sent_at"]
+            )
             send_grant_voucher_email.delay(grant_id=grant.id)
             return
 

backend/grants/tests/test_tasks.py

@@ -528,6 +528,30 @@ def test_send_grant_voucher_email(settings, sent_emails):
     )
 
 
+def test_send_grant_voucher_email_skips_when_already_sent(settings, sent_emails):
+    from notifications.models import EmailTemplateIdentifier
+    from notifications.tests.factories import EmailTemplateFactory
+
+    settings.FRONTEND_URL = "https://pycon.it"
+    user = UserFactory()
+    grant = GrantFactory(user=user, status=Grant.Status.confirmed)
+    ConferenceVoucherFactory(
+        user=user,
+        conference=grant.conference,
+        voucher_type=ConferenceVoucher.VoucherType.GRANT,
+        voucher_code="GRANT99",
+        voucher_email_sent_at=datetime(2020, 1, 1, tzinfo=timezone.utc),
+    )
+    EmailTemplateFactory(
+        conference=grant.conference,
+        identifier=EmailTemplateIdentifier.grant_voucher_code,
+    )
+
+    send_grant_voucher_email(grant_id=grant.id)
+
+    assert sent_emails().count() == 0
+
+
 def test_create_and_send_voucher_to_grantee(mocker):
     mock_create = mocker.patch(
         "conferences.vouchers.create_voucher", return_value={"id": 123}
@@ -626,6 +650,40 @@ def test_create_and_send_voucher_to_grantee_upgrades_co_speaker(mocker):
     mock_send_email.delay.assert_called_once_with(grant_id=grant.id)
 
 
+def test_create_and_send_voucher_to_grantee_upgrades_co_speaker_clears_email_sent_at(
+    mocker, settings, sent_emails
+):
+    from notifications.models import EmailTemplateIdentifier
+    from notifications.tests.factories import EmailTemplateFactory
+
+    settings.FRONTEND_URL = "https://pycon.it"
+    mock_create = mocker.patch("conferences.vouchers.create_voucher")
+    grant = GrantFactory(status=Grant.Status.confirmed)
+    prior_sent = datetime(2020, 5, 5, 12, 0, 0, tzinfo=timezone.utc)
+    ConferenceVoucherFactory(
+        conference=grant.conference,
+        user=grant.user,
+        voucher_type=ConferenceVoucher.VoucherType.CO_SPEAKER,
+        voucher_email_sent_at=prior_sent,
+    )
+    EmailTemplateFactory(
+        conference=grant.conference,
+        identifier=EmailTemplateIdentifier.grant_voucher_code,
+    )
+
+    create_and_send_voucher_to_grantee(grant_id=grant.id)
+
+    mock_create.assert_not_called()
+    voucher = ConferenceVoucher.objects.get(
+        conference=grant.conference,
+        user=grant.user,
+    )
+    assert voucher.voucher_type == ConferenceVoucher.VoucherType.GRANT
+    assert voucher.voucher_email_sent_at is not None
+    assert voucher.voucher_email_sent_at != prior_sent
+    assert sent_emails().count() == 1
+
+
 def test_create_and_send_voucher_to_grantee_creates_when_voucher_on_other_conference(
     mocker,
 ):