Use select+joins in a scope to speed up the #index action

app/controllers/organizations_controller.rb

@@ -6,7 +6,11 @@ class OrganizationsController < ApplicationController
   # GET /organizations
   # GET /organizations.json
   def index
-    @organizations = Organization.all
+    @organizations = if request.format.html?
+                       Organization.with_hq_names_and_store_count.all
+                     else
+                       Organization.with_hq_and_store_names.all
+                     end
   end
 
   # GET /organizations/1

app/models/organization.rb

@@ -3,4 +3,39 @@
 class Organization < ApplicationRecord
   has_many :hqs
   has_many :stores, through: :hqs
+
+  scope :with_hq_names_and_store_count, -> do
+    select(%(
+      organizations.*,
+      (SELECT array_agg(
+                hqs.name ||
+                ':' ||
+                countries.name ||
+                '(' ||
+                (SELECT count(id) FROM stores WHERE stores.hq_id = hqs.id) ||
+                ')'
+              )
+       FROM hqs
+       JOIN countries ON countries.id = hqs.country_id
+       WHERE hqs.organization_id = organizations.id
+      ) AS hq_data
+    ))
+  end
+
+  scope :with_hq_and_store_names, -> do
+    select(%(
+      organizations.*,
+      (SELECT array_agg(json_build_object(
+                'id', hqs.id,
+                'name', hqs.name,
+                'country_name', countries.name,
+                'store_count', (SELECT count(id) FROM stores WHERE stores.hq_id = hqs.id),
+                'store_list', (SELECT array_agg(json_build_object('id', id, 'name', name)) FROM stores WHERE stores.hq_id = hqs.id)
+              ))
+       FROM hqs
+       JOIN countries ON countries.id = hqs.country_id
+       WHERE hqs.organization_id = organizations.id
+      ) AS hq_data
+    ))
+  end
 end

app/views/organizations/_hq_list.html.erb

@@ -1,3 +1,3 @@
-<% organization.hqs.each do |hq| %>
-  <%= hq.name %>: <%= hq.country.name %> (<%= hq.stores.count %> stores) <br>
+<% organization.hq_data.each do |hq| %>
+  <%= hq %> <br>
 <% end %>

app/views/organizations/_organization.json.jbuilder

@@ -4,13 +4,11 @@ json.extract! organization, :id, :name, :address, :contact_name, :contact_phone,
 
 json.url organization_url(organization, format: :json)
 
-json.hqs organization.hqs do |hq|
-  json.extract! hq, :id, :name
+json.hqs organization.hq_data do |hq|
+  json.extract! hq, 'id', 'name'
 
-  json.country hq.country.name
-  json.store_count hq.stores.count
+  json.country hq['country_name']
+  json.store_count hq['store_count']
 
-  json.stores hq.stores do |store|
-    json.extract! store, :id, :name
-  end
+  json.stores hq['store_list']
 end