Skip CP change emails for users who have withdrawn all forecasts

When sending "Significant change" (CP change) emails, skip users who
have withdrawn their predictions from ALL questions in a post.

- Add helper function get_users_with_active_forecasts_for_questions()
  that returns user IDs with at least one active forecast
- Modify notify_post_cp_change() to skip users without active forecasts
- Add tests for the new functionality

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: palcu

posts/services/subscriptions.py

@@ -137,6 +137,25 @@ def get_last_user_forecasts_for_questions(
     return forecasts_map
 
 
+def get_users_with_active_forecasts_for_questions(
+    question_ids: Iterable[int],
+) -> set[int]:
+    """
+    Returns set of user IDs who have at least one active forecast
+    on any of the given questions.
+
+    An active forecast has end_time IS NULL or end_time > now().
+    """
+    return set(
+        Forecast.objects.filter(
+            question_id__in=question_ids,
+        )
+        .filter(Q(end_time__isnull=True) | Q(end_time__gt=timezone.now()))
+        .values_list("author_id", flat=True)
+        .distinct()
+    )
+
+
 def notify_post_cp_change(post: Post):
     """
     TODO: write description and check over
@@ -167,7 +186,16 @@ def notify_post_cp_change(post: Post):
         [q.pk for q in questions]
     )
 
+    # Get users who still have active forecasts on any question
+    users_with_active_forecasts = get_users_with_active_forecasts_for_questions(
+        [q.pk for q in questions]
+    )
+
     for subscription in subscriptions:
+        # Skip users who have withdrawn from all questions in the post
+        if subscription.user_id not in users_with_active_forecasts:
+            continue
+
         last_sent = subscription.last_sent_at
         max_sorting_diff = None
         question_data: list[CPChangeData] = []

tests/unit/test_posts/test_services/test_subscriptions.py

@@ -1,5 +1,6 @@
 from datetime import datetime, timedelta
 
+from django.utils import timezone
 from django.utils.timezone import make_aware
 from freezegun import freeze_time
 
@@ -9,9 +10,12 @@
     notify_new_comments,
     create_subscription_specific_time,
     notify_date,
+    get_users_with_active_forecasts_for_questions,
 )
 from tests.unit.test_comments.factories import factory_comment
 from tests.unit.test_posts.factories import factory_post
+from tests.unit.test_questions.factories import create_question, factory_forecast
+from questions.models import Question
 
 
 def test_notify_new_comments(user1, user2):
@@ -124,3 +128,70 @@ def test_notify_date__daily(self, user1):
         with freeze_time("2024-09-18T13:46Z"):
             notify_date()
             assert Notification.objects.filter(recipient=user1).count() == 2
+
+
+class TestGetUsersWithActiveForecasts:
+    def test_returns_users_with_active_forecasts(self, user1, user2):
+        """Users with active forecasts (end_time=None) should be returned"""
+        question = create_question(question_type=Question.QuestionType.BINARY)
+        factory_post(author=user1, question=question)
+
+        # User1 has an active forecast (end_time=None)
+        factory_forecast(author=user1, question=question)
+
+        result = get_users_with_active_forecasts_for_questions([question.pk])
+
+        assert user1.pk in result
+        assert user2.pk not in result
+
+    def test_excludes_users_with_withdrawn_forecasts(self, user1):
+        """Users with withdrawn forecasts (end_time set in past) should be excluded"""
+        question = create_question(question_type=Question.QuestionType.BINARY)
+        factory_post(author=user1, question=question)
+
+        # User1 has a withdrawn forecast (end_time in the past)
+        factory_forecast(
+            author=user1,
+            question=question,
+            end_time=timezone.now() - timedelta(hours=1),
+        )
+
+        result = get_users_with_active_forecasts_for_questions([question.pk])
+
+        assert user1.pk not in result
+
+    def test_includes_users_with_future_end_time(self, user1):
+        """Users with end_time in the future are still considered active"""
+        question = create_question(question_type=Question.QuestionType.BINARY)
+        factory_post(author=user1, question=question)
+
+        # User1 has a forecast that will be withdrawn in the future
+        factory_forecast(
+            author=user1,
+            question=question,
+            end_time=timezone.now() + timedelta(hours=1),
+        )
+
+        result = get_users_with_active_forecasts_for_questions([question.pk])
+
+        assert user1.pk in result
+
+    def test_user_with_one_active_one_withdrawn(self, user1):
+        """User with at least one active forecast should be included"""
+        question1 = create_question(question_type=Question.QuestionType.BINARY)
+        question2 = create_question(question_type=Question.QuestionType.BINARY)
+        factory_post(author=user1, question=question1)
+
+        # User1 has one withdrawn and one active forecast
+        factory_forecast(
+            author=user1,
+            question=question1,
+            end_time=timezone.now() - timedelta(hours=1),
+        )
+        factory_forecast(author=user1, question=question2)
+
+        result = get_users_with_active_forecasts_for_questions(
+            [question1.pk, question2.pk]
+        )
+
+        assert user1.pk in result