app/controllers/groups_controller.rb

@@ -2,7 +2,6 @@
 
 class GroupsController < ApplicationController
   def index
-    roots = Group.where(parent_id: nil)
+    render json: Groups::SummaryQuery.new.call.as_json
-    render jsonapi: roots, include: [children: [children: [:children]]]
   end
 end

app/models/group.rb

@@ -30,6 +30,8 @@ class Group < ApplicationRecord
   private
 
   def set_color
+    return if color.present?
+
     new_color = "##{SecureRandom.hex(3)}".paint
     new_color = new_color.lighten(30) if new_color.dark?
     self.color = new_color

app/queries/groups/summary_query.rb

@@ -0,0 +1,29 @@
+module Groups
+  class SummaryQuery
+    def call
+      ActiveRecord::Base.connection.execute(query).to_a
+    end
+
+    private
+
+    def query
+      <<~SQL.squish
+        SELECT#{' '}
+          groups.id,
+          groups.name,
+          groups.icon,
+          groups.parent_id,
+          groups.color,
+          count(*) filter (where status IS NOT NULL) as total,
+          count(*) filter (where status = 0) as considered,
+          count(*) filter (where status = 10) as invited,
+          count(*) filter (where status = 20) as confirmed,
+          count(*) filter (where status = 30) as declined,
+          count(*) filter (where status = 40) as tentative
+        FROM groups
+        LEFT JOIN guests on groups.id = guests.group_id
+        GROUP BY groups.id
+      SQL
+    end
+  end
+end

spec/queries/groups/summary_query_spec.rb

@@ -0,0 +1,96 @@
+require 'rails_helper'
+
+module Groups
+  RSpec.describe SummaryQuery do
+    describe '#call' do
+      subject { described_class.new.call }
+
+      context 'when there are no groups' do
+        it { is_expected.to eq([]) }
+      end
+
+      context 'when groups are defined' do
+        let!(:parent) { create(:group, name: 'Friends', icon: 'icon-1', color: '#FF0000') }
+        let!(:child) { create(:group, name: 'Family', icon: 'icon-2', color: '#00FF00', parent:) }
+
+        context 'when there are no guests' do
+          it 'returns the summary of groups' do
+            is_expected.to contain_exactly(
+              { 'id' => parent.id,
+                'name' => 'Friends',
+                'icon' => 'icon-1',
+                'parent_id' => nil,
+                'color' => '#FF0000',
+                'total' => 0,
+                'considered' => 0,
+                'invited' => 0,
+                'confirmed' => 0,
+                'declined' => 0,
+                'tentative' => 0 },
+              { 'id' => child.id,
+                'name' => 'Family',
+                'icon' => 'icon-2',
+                'parent_id' => parent.id,
+                'color' => '#00FF00',
+                'total' => 0,
+                'considered' => 0,
+                'invited' => 0,
+                'confirmed' => 0,
+                'declined' => 0,
+                'tentative' => 0 }
+            )
+          end
+        end
+
+        context 'when there are guests' do
+          before do
+            # Parent group
+            create_list(:guest, 2, group: parent, status: :considered)
+            create_list(:guest, 3, group: parent, status: :invited)
+            create_list(:guest, 4, group: parent, status: :confirmed)
+            create_list(:guest, 5, group: parent, status: :declined)
+            create_list(:guest, 6, group: parent, status: :tentative)
+
+            # Child group
+            create_list(:guest, 7, group: child, status: :considered)
+            create_list(:guest, 8, group: child, status: :invited)
+            create_list(:guest, 9, group: child, status: :confirmed)
+            create_list(:guest, 10, group: child, status: :declined)
+            create_list(:guest, 11, group: child, status: :tentative)
+          end
+
+          it 'returns the summary of groups' do
+            is_expected.to contain_exactly(
+              {
+                'id' => parent.id,
+                'name' => 'Friends',
+                'icon' => 'icon-1',
+                'parent_id' => nil,
+                'color' => '#FF0000',
+                'total' => 20,
+                'considered' => 2,
+                'invited' => 3,
+                'confirmed' => 4,
+                'declined' => 5,
+                'tentative' => 6
+              },
+              {
+                'id' => child.id,
+                'name' => 'Family',
+                'icon' => 'icon-2',
+                'parent_id' => parent.id,
+                'color' => '#00FF00',
+                'total' => 45,
+                'considered' => 7,
+                'invited' => 8,
+                'confirmed' => 9,
+                'declined' => 10,
+                'tentative' => 11
+              }
+            )
+          end
+        end
+      end
+    end
+  end
+end