Bound Activity loads on ActivityParticipation admin forms

[thibaudgg]

Problem

ActivityParticipationsController#edit (and #new, same ActiveAdmin form) got slower in production because the activity dropdown loaded every past Activity with SELECT * and no LIMIT.

AppSignal CSA Admin / production, Europe/Zurich week 7–14 Sep 2026:

• Action: ActivityParticipationsController#edit
• Regression vs prior week: p95 ~222ms → ~408ms (n≈27 this week)
• Slow traces 0.65–1.9s across tenants; all flagged N+1
• Worst cluster: tenant clefdeschamps, rev 6de5f427…, ~1.8–1.9s on 12 Sep ~15:21 CEST

Trace 13f9da2263b7dce15f02c3d0c0f26b97 (1899.9ms):

• Layout/form dominate: active_admin/resource/edit → _form ≈1719ms
• Hotspot SQL (no LIMIT):

SELECT "activities".* FROM "activities" WHERE "activities"."date" < ? ORDER BY "activities"."date" DESC

• That Activity Load was ~69ms SQL + ~370ms ActiveRecord instantiation with ~94k allocations

Coming activities used the same unbounded SELECT * (cheap on that tenant). A follow-up WHERE id = ? LIMIT ? Activity Load and the member select remain; those are single-row / needed dropdown data, not the hotspot.

Change

The form called grouped_by_date(Activity), which instantiates the full past (and coming) relation for the select.

It now uses Activity.admin_form_collection:

• Coming and past groups are each LIMIT 200
• Only label columns are selected (id, date, start_time, end_time, places)
• An already-assigned activity outside that window stays in the select so edit stays usable
• UI is unchanged: Coming / Past optgroups, same labels

No new frontend stack. Member collection is unchanged (needed for the member select; ~4.5ms in the trace).

Tests

Controller and model tests assert the form Activity queries stay bounded as activity count grows (LIMIT, no SELECT *, no descriptions / titles / place_urls), and that an assigned activity older than the limit remains selected.

Locally: bin/rails test test/models/activity_test.rb test/controllers/activity_participations_controller_test.rb and RuboCop on the touched files are green.