fix(backend): validate Display start_at must not exceed expires_at in Announcement ($25 bounty proposal)

backend/models/announcement.py

@@ -2,7 +2,7 @@
 from enum import Enum
 from typing import Any, List, Mapping, Optional, TypeVar, cast
 
-from pydantic import BaseModel, ValidationError, field_validator
+from pydantic import BaseModel, ValidationError, field_validator, model_validator
 
 
 def _ensure_utc(value: Optional[datetime]) -> Optional[datetime]:
@@ -74,6 +74,12 @@ class Display(BaseModel):
     def _normalize_display_datetimes(cls, v: Optional[datetime]) -> Optional[datetime]:
         return _ensure_utc(v)
 
+    @model_validator(mode="after")
+    def validate_bounds(self):
+        if self.start_at is not None and self.expires_at is not None and self.start_at > self.expires_at:
+            raise ValueError("display start_at must not exceed expires_at")
+        return self
+
     def to_dict(self) -> dict[str, Any]:
         return {
             "priority": self.priority,

backend/tests/unit/test_announcement_display_bounds.py

@@ -0,0 +1,55 @@
+import unittest
+from datetime import datetime, timezone
+from pydantic import ValidationError
+
+from models.announcement import Announcement, Display
+
+
+class TestDisplayBoundsValidation(unittest.TestCase):
+    def test_display_valid_dates(self):
+        start = datetime(2026, 1, 1, 0, 0, 0, tzinfo=timezone.utc)
+        expires = datetime(2026, 1, 2, 0, 0, 0, tzinfo=timezone.utc)
+        disp = Display(start_at=start, expires_at=expires)
+        self.assertEqual(disp.start_at, start)
+        self.assertEqual(disp.expires_at, expires)
+
+    def test_display_equal_start_and_expires(self):
+        ts = datetime(2026, 1, 1, 12, 0, 0, tzinfo=timezone.utc)
+        disp = Display(start_at=ts, expires_at=ts)
+        self.assertEqual(disp.start_at, ts)
+        self.assertEqual(disp.expires_at, ts)
+
+    def test_display_only_start_or_expires(self):
+        ts = datetime(2026, 1, 1, 12, 0, 0, tzinfo=timezone.utc)
+        disp1 = Display(start_at=ts)
+        self.assertEqual(disp1.start_at, ts)
+        self.assertIsNone(disp1.expires_at)
+
+        disp2 = Display(expires_at=ts)
+        self.assertIsNone(disp2.start_at)
+        self.assertEqual(disp2.expires_at, ts)
+
+    def test_display_start_after_expires_raises_validation_error(self):
+        start = datetime(2026, 1, 5, 0, 0, 0, tzinfo=timezone.utc)
+        expires = datetime(2026, 1, 1, 0, 0, 0, tzinfo=timezone.utc)
+        with self.assertRaises(ValidationError) as ctx:
+            Display(start_at=start, expires_at=expires)
+        self.assertIn("display start_at must not exceed expires_at", str(ctx.exception))
+
+    def test_announcement_from_dict_drops_inverted_display_subdoc(self):
+        doc = {
+            "id": "ann-inverted-display",
+            "type": "announcement",
+            "created_at": "2026-01-01T00:00:00Z",
+            "display": {
+                "start_at": "2026-01-10T00:00:00Z",
+                "expires_at": "2026-01-05T00:00:00Z",
+            },
+        }
+        ann = Announcement.from_dict(doc)
+        self.assertEqual(ann.id, "ann-inverted-display")
+        self.assertIsNone(ann.display)
+
+
+if __name__ == "__main__":
+    unittest.main()