⚡ Stop the events homepage from running a query per event

[jefftriplett]

/events/ runs a query for every event on the page.

EventHomepage loads all past and all upcoming events into Python lists, and the list template then reads next_time, is_scheduled_to_start_this_year, is_scheduled_to_end_this_year, venue and get_absolute_url for each row, and every one of those hits the database.

Two changes:

• The homepage querysets now select_related("occurring_rule", "venue", "calendar") and prefetch_related("recurring_rules").
• Event.next_time / Event.previous_time filter the prefetched recurring_rules in Python. They called .filter() on the related manager, which bypasses the prefetch cache and so queried once per event.

GET /events/, locally against Postgres:

events	before	after
90	638 queries, 0.19s	28 queries, 0.05s
2000	8362 queries, 4.17s	28 queries, 0.48s

Fixes #3125

[Copilot]

Copilot review overview

🟡 Changes recommended

There are a few correctness/performance issues in the updated code paths (notably an incorrect return annotation and a potential performance regression when recurring_rules is not prefetched) that should be addressed before merging.

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Review effort: Lite
Findings: 2 · 1

Open (3)

• Avoid loading all recurring rules in next_time · New
• Compute previous_time once per sort key · New
• Incorrect get_queryset return annotation · New

What changed in this PR

Reduces database query volume on the /events/ homepage by ensuring the list view eagerly loads related event data and by making Event.next_time / Event.previous_time able to reuse prefetched recurring_rules rather than triggering per-event queries during template rendering.

Changes:

• Add a with_related() helper on EventHomepage and apply select_related(...) / prefetch_related(...) to the homepage querysets.
• Update Event.next_time / Event.previous_time to filter recurring_rules in Python so the prefetch cache can be reused.
• Add a regression test asserting homepage query count does not grow as more events are added.

File	Description
apps/​events/​views.py	Centralizes related-object eager loading for the events homepage and reuses a single now timestamp across homepage queries/sorts.
apps/​events/​models.py	Adjusts next_time / previous_time to avoid bypassing prefetched recurring_rules cache.
apps/​events/​tests/​test_views.py	Adds a query-count regression test to guard against future N+1 regressions on /events/.

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[Copilot]

Copilot review overview

🟢 Approval recommended

The changes directly address the N+1 query behavior with consistent eager-loading and include a regression test guarding against query-per-row regressions.

Review effort: Lite
Findings: None

Resolved since last review (3)

• Compute previous_time once per sort key
• Avoid loading all recurring rules in next_time
• Incorrect get_queryset return annotation

[Copilot]

Copilot review overview

🟢 Approval recommended

The changes directly address the documented N+1 query cause, add targeted eager-loading, and include regression tests to prevent performance and rendering regressions.

Review effort: Lite
Findings: None

[Copilot]

Copilot review overview

🟢 Approval recommended

The changes are targeted, consistent across call sites, and include a regression test specifically guarding against the original query-per-row performance issue.

Review effort: Lite
Findings: None