fix: eliminate N+1 queries in EventsController#index

app/controllers/events_controller.rb

@@ -8,19 +8,25 @@ class EventsController < ApplicationController
   def index
     fresh_when(latest_model_updated, etag: latest_model_updated)
 
-    events = [Workshop.past.includes(:chapter,
-                                     :sponsors).joins(:chapter).merge(Chapter.active).limit(RECENT_EVENTS_DISPLAY_LIMIT)]
-    events << Meeting.past.includes(:venue).limit(RECENT_EVENTS_DISPLAY_LIMIT)
-    events << Event.past.includes(:venue, :sponsors, :sponsorships).limit(RECENT_EVENTS_DISPLAY_LIMIT)
+    events = [Workshop.past
+                      .includes(:chapter, :sponsors, :host, :permissions)
+                      .joins(:chapter)
+                      .merge(Chapter.active)
+                      .limit(RECENT_EVENTS_DISPLAY_LIMIT)]
+    events << Meeting.past.includes(:venue, :permissions).limit(RECENT_EVENTS_DISPLAY_LIMIT)
+    events << Event.past.includes(:venue, :sponsors, :sponsorships, :permissions).limit(RECENT_EVENTS_DISPLAY_LIMIT)
     events = events.compact.flatten.sort_by(&:date_and_time).reverse.first(RECENT_EVENTS_DISPLAY_LIMIT)
     events_hash_grouped_by_date = events.group_by(&:date)
     @past_events = events_hash_grouped_by_date.map.each_with_object({}) do |(key, value), hash|
       hash[key] = EventPresenter.decorate_collection(value)
     end
 
-    events = [Workshop.includes(:chapter, :sponsors).upcoming.joins(:chapter).merge(Chapter.active)]
-    events << Meeting.upcoming.all
-    events << Event.upcoming.includes(:venue, :sponsors, :sponsorships).all
+    events = [Workshop.upcoming
+                      .includes(:chapter, :sponsors, :host, :permissions)
+                      .joins(:chapter)
+                      .merge(Chapter.active)]
+    events << Meeting.upcoming.includes(:venue, :permissions).all
+    events << Event.upcoming.includes(:venue, :sponsors, :sponsorships, :permissions).all
     events = events.compact.flatten.sort_by(&:date_and_time).group_by(&:date)
     @events = events.map.each_with_object({}) do |(key, value), hash|
  hash[key] = EventPresenter.decorate_collection(value)

app/models/workshop.rb

@@ -10,6 +10,10 @@ class Workshop < ApplicationRecord
   has_many :invitations, class_name: 'WorkshopInvitation'
   has_many :workshop_sponsors
   has_many :sponsors, through: :workshop_sponsors
+  has_one :workshop_host, -> { where(workshop_sponsors: { host: true }) },
+          class_name: 'WorkshopSponsor',
+          inverse_of: :workshop
+  has_one :host, through: :workshop_host, source: :sponsor
   has_many :organisers, -> { where('permissions.name' => 'organiser') }, through: :permissions, source: :members
   has_many :feedbacks
 
@@ -31,18 +35,7 @@ class Workshop < ApplicationRecord
   before_validation :set_opens_at
 
   def host
-    sql = <<~SQL
-      SELECT sponsors.*
-      FROM sponsors
-      LEFT JOIN workshop_sponsors ON workshop_sponsors.sponsor_id = sponsors.id
-      WHERE workshop_sponsors.workshop_id = ?
-        AND workshop_sponsors.host = TRUE
-        AND sponsors.id = workshop_sponsors.sponsor_id
-      ORDER BY sponsors.updated_at DESC, workshop_sponsors.id ASC
-      LIMIT 1
-    SQL
-
-    Sponsor.find_by_sql([sql, id]).first
+    workshop_host&.sponsor
   end
 
   def waiting_list

app/models/workshop_sponsor.rb

@@ -1,6 +1,6 @@
 class WorkshopSponsor < ApplicationRecord
   belongs_to :sponsor
-  belongs_to :workshop
+  belongs_to :workshop, inverse_of: :workshop_sponsors
 
   validates :sponsor_id, uniqueness: { scope: :workshop_id, message: :already_sponsoring }
 end

app/presenters/workshop_presenter.rb

@@ -28,7 +28,7 @@ def attending_and_available_coach_spots
   end
 
   def venue
-    model.host
+    @venue ||= model.host
   end
 
   def organisers

db/migrate/20260224130000_add_index_workshop_sponsors_host.rb

@@ -0,0 +1,5 @@
+class AddIndexWorkshopSponsorsHost < ActiveRecord::Migration[8.1]
+  def change
+    add_index :workshop_sponsors, %i[workshop_id host], name: 'index_workshop_sponsors_on_workshop_id_and_host'
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.1].define(version: 2026_02_24_120000) do
+ActiveRecord::Schema[8.1].define(version: 2026_02_24_130000) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_catalog.plpgsql"
 
@@ -565,6 +565,7 @@
     t.datetime "updated_at", precision: nil
     t.integer "workshop_id"
     t.index ["sponsor_id"], name: "index_workshop_sponsors_on_sponsor_id"
+    t.index ["workshop_id", "host"], name: "index_workshop_sponsors_on_workshop_id_and_host"
     t.index ["workshop_id"], name: "index_workshop_sponsors_on_workshop_id"
   end