Performance/n+1 query ical export

[Tschuppi81]

Event: Fixes the N+1 query by dropping eagerly loading on the unused relationships in the iCal export

TYPE: Performance
LINK: https://seantis-gmbh.sentry.io/issues/7605741267

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.49%. Comparing base (2794281) to head (8131582).
✅ All tests successful. No failed tests found.

Additional details and impacted files

Files with missing lines	Coverage Δ
src/onegov/event/collections/occurrences.py	97.40% <100.00%> (+0.01%)	⬆️

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[Daverball]

Technically a false positive, since it's more like a N/500 + 1 query, it appears that SQLAlchemy loads the related objects in batches of 500 for the selectin strategy.

But since we actually don't need the occurrences here, it doesn't hurt to skip loading them. But I would probably switch to noload or raiseload from lazyload though, so we don't get a real N+1 query in the future, if we do end up accessing the occurrences for something. We'd rather know about this performance regression early, by virtue of our tests failing.