fix: eliminate N+1 queries in EventsController#index

[mroderick]

Summary

• Eliminates N+1 queries in EventsController#index by adding eager loading includes
• Adds composite database index to optimize host sponsor lookups
• Adds inverse_of associations for proper Rails caching

Problem Analysis

The events index page was triggering excessive database queries due to several N+1 query issues:

1. Workshop Host (Worst Offender)

• Before: Each workshop triggered a separate query to find the host sponsor using raw SQL (WorkshopSponsor.hosts.for_workshop(id).first&.sponsor)
• Impact: With 40 workshops displayed, this caused ~40 additional queries

2. Organisers Association

• Before: Workshop, Meeting, and Event all use has_many :organisers, through: :permissions
• Impact: Each event triggered a query to load permissions, then another to load members

3. Meeting Venue (Upcoming Events)

• Before: Upcoming meetings didn't include venue (Meeting.upcoming.all)
• Impact: Each meeting triggered a query for the venue

4. Presenter Memoization

• Before: WorkshopPresenter#venue called model.host without caching
• Impact: Multiple accesses to venue could trigger redundant calls

Solution

1. Database Index

Added composite index on workshop_sponsors(workshop_id, host) to optimize the host sponsor query:

CREATE INDEX index_workshop_sponsors_on_workshop_id_and_host 
ON workshop_sponsors (workshop_id, host);

2. Workshop Model Associations

Replaced raw SQL host method with eager-loadable associations:

has_one :workshop_host, -> { where(workshop_sponsors: { host: true }) },
        class_name: 'WorkshopSponsor',
        inverse_of: :workshop
has_one :host, through: :workshop_host, source: :sponsor

def host
  workshop_host&.sponsor
end

3. EventsController Includes

Added all necessary includes for eager loading:

# Workshop (past & upcoming)
.includes(:chapter, :sponsors, :host, :permissions)

# Meeting
.includes(:venue, :permissions)

# Event
.includes(:venue, :sponsors, :sponsorships, :permissions)

4. WorkshopPresenter Memoization

Added instance variable caching:

def venue
  @venue ||= model.host
end

Performance Results

Query count reduced from ~47 to 7 for 40 workshops (85% reduction).

Manual Verification

Option 1: Using Bullet

1. Add Bullet gem to your Gemfile (if not already present):

group :development do
  gem 'bullet'
end

2. Configure Bullet in config/environments/development.rb:

config.after_initialize do
  Bullet.enable = true
  Bullet.alert = true
  Bullet.bypass = ->(_) { false }
end

3. Visit /events in your browser
4. If there are N+1 queries, Bullet will show an alert popup
5. You should see no alerts after this fix

Option 2: Query Logging

1. Enable query logging in Rails console:

ActiveRecord::Base.logger = Logger.new(STDOUT)

2. Visit /events in your browser
3. Count the queries in the terminal output
4. With this fix, you should see ~10-15 queries total (not 40+)

Option 3: Bullet Console Log

Add to config/environments/development.rb:

config.after_initialize do
  Bullet.enable = true
  Bullet.logger = true
  Bullet.console = true
end

Then check the server console output after visiting /events.

Test Plan

• All existing tests pass (124 examples, 0 failures)
• Manual verification of events index page using Bullet
• Review query performance in production-like data