Speed up /coaches: partial index and database-side coach count

app/controllers/dashboard_controller.rb

@@ -31,7 +31,7 @@ def faq; end
   def about; end
 
   def wall_of_fame
-    @coaches_count = top_coach_query.length
+    @coaches_count = WorkshopInvitation.to_coaches.attended.distinct.count(:member_id)
     coaches = Member.where(id: top_coach_query
                                .year(year_param))
                     .includes(:skills)

db/migrate/20260917090000_add_coach_attended_index_to_workshop_invitations.rb

@@ -0,0 +1,16 @@
+class AddCoachAttendedIndexToWorkshopInvitations < ActiveRecord::Migration[8.1]
+  disable_ddl_transaction!
+
+  def up
+    add_index :workshop_invitations, :member_id,
+              where: "attended AND role = 'Coach'",
+              name: :index_workshop_invitations_coach_attended_on_member_id,
+              algorithm: :concurrently
+  end
+
+  def down
+    remove_index :workshop_invitations,
+                 name: :index_workshop_invitations_coach_attended_on_member_id,
+                 algorithm: :concurrently
+  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_09_17_023327) do
+ActiveRecord::Schema[8.1].define(version: 2026_09_17_090000) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_catalog.plpgsql"
 
@@ -615,6 +615,7 @@
     t.integer "workshop_id"
     t.index ["member_id", "attending"], name: "index_workshop_invitations_member_attending"
     t.index ["member_id", "workshop_id", "role"], name: "idx_on_member_id_workshop_id_role_e3cea6bbfd", unique: true
+    t.index ["member_id"], name: "index_workshop_invitations_coach_attended_on_member_id", where: "(attended AND ((role)::text = 'Coach'::text))"
     t.index ["member_id"], name: "index_workshop_invitations_on_member_id"
     t.index ["token"], name: "index_workshop_invitations_on_token", unique: true
     t.index ["workshop_id", "attending"], name: "index_workshop_invitations_workshop_attending"

spec/features/listing_coaches_spec.rb

@@ -9,6 +9,19 @@
     expect(page).to have_text(coach.name, wait: 5)
   end
 
+  scenario 'I can see the total number of volunteers across all years' do
+    workshop_last_year = Fabricate(:workshop, date_and_time: 1.year.ago)
+    workshop_three_years_ago = Fabricate(:workshop, date_and_time: 3.years.ago)
+    coach = Fabricate(:attended_coach, workshop: workshop_last_year).member
+    Fabricate(:attended_coach, member: coach, workshop: workshop_three_years_ago)
+    Fabricate(:attended_coach, workshop: workshop_three_years_ago)
+
+    visit coaches_path
+
+    # The same coach at two workshops counts once
+    expect(page).to have_text('the 2 volunteers', wait: 5)
+  end
+
   scenario 'I can see the top coaches by year' do
     travel_to(Time.current) do
       latest_workshop = Fabricate(:workshop, date_and_time: 1.year.ago)