Cache the rendered /sponsors page body

app/controllers/sponsors_controller.rb

@@ -1,5 +1,13 @@
 class SponsorsController < ApplicationController
   def index
-    @sponsor_levels = Sponsor.active.group_by(&:level)
+    # v1: bump when the sponsors view or partials change, otherwise a deploy
+    # keeps serving the cached body until the next sponsor save.
+    key = "sponsors/index/v1/#{Sponsor.active.maximum(:updated_at)&.to_fs(:usec)}"
+    body = Rails.cache.fetch(key) do
+      @sponsor_levels = Sponsor.active.group_by(&:level)
+      render_to_string(layout: false)
+    end
+    # body is markup rendered by this app's own template, not user input
+    render html: body.html_safe # rubocop:disable Rails/OutputSafety
   end
 end

spec/requests/sponsors_spec.rb

@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe 'Sponsors' do
+  let!(:sponsor) { Fabricate.create(:sponsor, name: 'Acme Corp') }
+
+  around do |example|
+    original_cache = Rails.cache
+    Rails.cache = ActiveSupport::Cache::MemoryStore.new
+    example.run
+    Rails.cache = original_cache
+  end
+
+  it 'renders the sponsors page' do
+    get '/sponsors'
+
+    expect(response).to have_http_status(:ok)
+    expect(response.body).to include('Acme Corp')
+  end
+
+  it 'serves the cached body when no sponsor has been updated' do
+    get '/sponsors'
+
+    # update_columns bypasses callbacks, so updated_at (and the cache key) stays unchanged
+    sponsor.update_columns(name: 'Renamed Corp')
+
+    get '/sponsors'
+
+    expect(response.body).to include('Acme Corp')
+    expect(response.body).not_to include('Renamed Corp')
+  end
+
+  it 're-renders when a sponsor is updated' do
+    get '/sponsors'
+
+    sponsor.update!(name: 'Renamed Corp')
+
+    get '/sponsors'
+
+    expect(response.body).to include('Renamed Corp')
+  end
+end