Ensure calendar slugs are unique

calmerge/config.py

@@ -80,6 +80,18 @@ def validate_offset_days(cls, offset_days: list[int]) -> list[int]:
 class Config(BaseModel):
     calendars: list[CalendarConfig] = Field(alias="calendar", default_factory=list)
 
+    @field_validator("calendars")
+    @classmethod
+    def validate_unique_calendar_slugs(
+        cls, calendars: list[CalendarConfig]
+    ) -> list[CalendarConfig]:
+        calendar_slugs = {calendar.slug for calendar in calendars}
+
+        if len(calendar_slugs) != len(calendars):
+            raise PydanticCustomError("calendar_slugs", "Calendar slugs must be unique")
+
+        return calendars
+
     @classmethod
     def from_file(cls, path: Path) -> "Config":
         with path.open(mode="rb") as f:

tests/test_config.py

@@ -42,6 +42,15 @@ def test_invalid_offset_days() -> None:
     )
 
 
+def test_duplicate_calendar_slug() -> None:
+    calendar_config = CalendarConfig(slug="test", urls=["https://example.com"])  # type: ignore [list-item]
+
+    with pytest.raises(ValidationError) as e:
+        Config(calendar=[calendar_config] * 5)
+
+    assert e.value.errors()[0]["msg"] == "Calendar slugs must be unique"
+
+
 def test_urls_expand_env_var(monkeypatch: pytest.MonkeyPatch) -> None:
     monkeypatch.setenv("FOO", "BAR")