app/controllers/expenses_controller.rb

@@ -8,6 +8,10 @@ class ExpensesController < ApplicationController
     @expenses = Expense.all
   end
 
+  def summary
+    render json: Expenses::TotalQuery.new.call
+  end
+
   # GET /expenses/1 or /expenses/1.json
   def show
   end

app/queries/expenses/total_query.rb

@@ -0,0 +1,48 @@
+# Copyright (C) 2024 Manuel Bustillo
+
+module Expenses
+  class TotalQuery
+    def call
+      ActiveRecord::Base.connection.execute(query).first
+    end
+
+    private
+
+    def query
+      <<~SQL
+        WITH guest_count AS (#{guest_count_per_status}),
+             expense_summary AS (#{expense_summary})
+        SELECT expense_summary.fixed,
+               expense_summary.fixed_count,
+               expense_summary.variable,
+               expense_summary.variable_count,
+               expense_summary.total_count,
+               guest_count.confirmed as confirmed_guests,
+                guest_count.projected as projected_guests,
+               expense_summary.fixed + expense_summary.variable * guest_count.confirmed as total,
+               expense_summary.fixed + expense_summary.variable * guest_count.projected as max_projected,
+               (expense_summary.fixed + expense_summary.variable * guest_count.confirmed) / guest_count.confirmed as per_person
+        FROM guest_count, expense_summary;
+      SQL
+    end
+
+    def expense_summary
+      <<~SQL
+        SELECT coalesce(sum(amount) filter (where pricing_type = 'fixed'), 0) as fixed,
+               coalesce(count(amount) filter (where pricing_type = 'fixed'), 0) as fixed_count,
+               coalesce(sum(amount) filter (where pricing_type = 'per_person'), 0) as variable,
+               coalesce(count(amount) filter (where pricing_type = 'per_person'), 0) as variable_count,
+               count(*) as total_count
+        FROM expenses
+      SQL
+    end
+
+    def guest_count_per_status
+      <<~SQL
+        SELECT COALESCE(count(*) filter(where status = #{Guest.statuses["confirmed"]}), 0) as confirmed,
+               COALESCE(count(*) filter(where status IN (#{Guest.statuses.values_at("confirmed", "invited", "tentative").join(",")})), 0) as projected
+        FROM guests
+      SQL
+    end
+  end
+end

config/routes.rb

@@ -6,7 +6,9 @@ Rails.application.routes.draw do
     post :import, on: :collection
     post :bulk_update, on: :collection
   end
-  resources :expenses
+  resources :expenses do
+    get :summary, on: :collection
+  end
   resources :tables_arrangements, only: [:index, :show]
 
   get 'up' => 'rails/health#show', as: :rails_health_check

spec/factories/expense.rb

@@ -0,0 +1,16 @@
+FactoryBot.define do
+    factory :expense do
+      sequence(:name) { |i| "Expense #{i}" }
+      pricing_type { "fixed" }
+      amount { 100 }
+    end
+
+    trait :fixed do
+      pricing_type { "fixed" }
+    end
+
+    trait :per_person do
+      pricing_type { "per_person" }
+    end
+  end
+  

spec/queries/expenses/total_query_spec.rb

@@ -0,0 +1,92 @@
+require 'rails_helper'
+
+module Expenses
+  RSpec.describe TotalQuery do
+    describe '#call' do
+      let(:response) { described_class.new.call }
+
+      before do
+        create_list(:guest, 2, status: :confirmed)
+        create_list(:guest, 3, status: :considered)
+        create_list(:guest, 4, status: :invited)
+        create_list(:guest, 5, status: :tentative)
+        create_list(:guest, 6, status: :declined)
+      end
+
+      context "when there is no expense" do
+        it "returns zero in all values", :aggregate_failures do
+          expect(response["fixed"]).to be_zero
+          expect(response["fixed_count"]).to be_zero
+          expect(response["variable"]).to be_zero
+          expect(response["variable_count"]).to be_zero
+          expect(response["total"]).to be_zero
+          expect(response["total_count"]).to be_zero
+          expect(response["max_projected"]).to be_zero
+          expect(response["per_person"]).to be_zero
+          expect(response["confirmed_guests"]).to eq(2)
+          expect(response["projected_guests"]).to eq(2 + 4 + 5)
+        end
+      end
+
+      context "when there are only fixed expenses" do
+        before do
+          create(:expense, :fixed, amount: 100)
+          create(:expense, :fixed, amount: 200)
+        end
+
+        it "returns the sum of fixed expenses", :aggregate_failures do
+          expect(response["fixed"]).to eq(300)
+          expect(response["fixed_count"]).to eq(2)
+          expect(response["variable"]).to be_zero
+          expect(response["variable_count"]).to be_zero
+          expect(response["total"]).to eq(300)
+          expect(response["total_count"]).to eq(2)
+          expect(response["max_projected"]).to eq(300)
+          expect(response["per_person"]).to eq(150)
+          expect(response["confirmed_guests"]).to eq(2)
+          expect(response["projected_guests"]).to eq(2 + 4 + 5)
+        end
+      end
+
+      context "when there are only variable expenses" do
+        before do
+          create(:expense, :per_person, amount: 100)
+          create(:expense, :per_person, amount: 200)
+        end
+
+        it "returns zero in the values and nonzero in the count", :aggregate_failures do
+          expect(response["fixed"]).to be_zero
+          expect(response["fixed_count"]).to be_zero
+          expect(response["variable"]).to eq(300)
+          expect(response["variable_count"]).to eq(2)
+          expect(response["total"]).to eq(2*300)
+          expect(response["total_count"]).to eq(2)
+          expect(response["max_projected"]).to eq(11*300)
+          expect(response["confirmed_guests"]).to eq(2)
+          expect(response["projected_guests"]).to eq(2 + 4 + 5)
+        end
+      end
+
+      context "when there are both fixed and variable expenses" do
+        before do
+          create(:expense, :fixed, amount: 100)
+          create(:expense, :fixed, amount: 200)
+          create(:expense, :per_person, amount: 50)
+        end
+
+        it "returns the sum of fixed and variable expenses", :aggregate_failures do
+          expect(response["fixed"]).to eq(300)
+          expect(response["fixed_count"]).to eq(2)
+          expect(response["variable"]).to eq(50)
+          expect(response["variable_count"]).to eq(1)
+          expect(response["total"]).to eq(100 + 200 + 50 * 2)
+          expect(response["total_count"]).to eq(3)
+          expect(response["max_projected"]).to eq(100 + 200 + 11*50)
+          expect(response["per_person"]).to eq(200)
+          expect(response["confirmed_guests"]).to eq(2)
+          expect(response["projected_guests"]).to eq(2 + 4 + 5)
+        end
+      end
+    end
+  end
+end