fix: homepage performance - missing indexes and inefficient queries

[mroderick]

Summary

Fixes the 503 timeout on the homepage (Heroku 30s timeout) caused by performance issues introduced in #2503.

Root Cause Analysis

Problem 1: Missing Database Indexes

The code changes in #2503 introduced new methods that query invitations filtered by attending: true:

# Member#attending_event_ids makes 3 queries:
invitations.accepted.pluck(:event_id)
workshop_invitations.accepted.pluck(:workshop_id)
meeting_invitations.accepted.pluck(:meeting_id)

The accepted scope filters on the attending column, but there were no indexes on (member_id, attending) or (event_id, attending), causing full table scans.

Problem 2: Inefficient total_upcoming_events_count

# BEFORE: Loads ALL workshops with full eager loading just to count
def total_upcoming_events_count
  workshops = Workshop.eager_load(:chapter, :sponsors, :organisers, :permissions)
  all_events(workshops.to_a).count  # Loads EVERYTHING, then counts
end

This loaded ALL workshops in the database with expensive JOINs (including polymorphic permissions) just to get a count.

Problem 3: Presenters Bypass Eager Loading

The dashboard queries workshops with eager_load(:permissions), but the presenters ignored this and made their own queries:

# EventPresenter#organisers - bypasses eager loading
def organisers
  model.permissions.find_by(name: 'organiser')&.members || []
end

This triggered additional queries per event, defeating the purpose of eager loading.

Fixes Applied

1. Database Migration

Added composite indexes for invitation queries:

add_index :workshop_invitations, [:member_id, :attending]
add_index :workshop_invitations, [:workshop_id, :attending]
add_index :meeting_invitations, [:member_id, :attending]
add_index :meeting_invitations, [:meeting_id, :attending]
add_index :invitations, [:member_id, :attending]
add_index :invitations, [:event_id, :attending]

2. Optimized Event Counter

# AFTER: Simple COUNT queries
def total_upcoming_events_count
  workshop_count = Workshop.upcoming.count
  event_count = Event.upcoming.count
  meeting_exists = Meeting.next.present? ? 1 : 0
  workshop_count + event_count + meeting_exists
end

3. Use Eager-Loaded Associations

# EventPresenter#organisers - now uses pre-loaded data
def organisers
  @organisers ||= model.organisers.to_a
end

The presenters now use the model.organisers association (which uses the eager-loaded permissions) instead of making additional queries.

Testing

• All existing tests pass (77 presenter tests, 8 homepage feature tests)
• Migration applied successfully to test database

Deployment

Run the migration on production:

bundle exec rails db:migrate

[olleolleolle]

Nice to only count those! And to get the presenters fixed!