# Invoice System Implementation Guide

## Overview

This document outlines various approaches for adding invoice generation to the ApéroNight event ticketing system, based on the current Rails architecture with Stripe integration.

## Current System Analysis

### Existing Infrastructure
- **Rails 8.0.2** with MySQL database
- **Stripe** payment processing via Checkout Sessions
- **Prawn** gem for PDF generation (tickets)
- **Order/Ticket** models with pricing logic
- **Devise** for user authentication

### Current Payment Flow
```
User selects tickets → Order created (draft) → Stripe Checkout → Payment → Order marked as paid → Tickets activated → PDF tickets emailed
```

## Invoice Generation Approaches

### 1. Extend Existing PDF Infrastructure (Recommended)

**Pros**: 
- Leverages existing Prawn setup
- Consistent styling with tickets
- No additional dependencies

**Implementation**:
```ruby
# app/services/invoice_pdf_generator.rb
class InvoicePdfGenerator
  attr_reader :order

  def initialize(order)
    @order = order
  end

  def generate
    Prawn::Document.new(page_size: "A4", margin: 40) do |pdf|
      # Header
      pdf.fill_color "2D1B69"
      pdf.font "Helvetica", style: :bold, size: 24
      pdf.text "ApéroNight Invoice", align: :center
      pdf.move_down 20

      # Invoice details
      pdf.fill_color "000000"
      pdf.font "Helvetica", size: 12
      
      # Invoice number and date
      pdf.text "Invoice #: #{@order.invoice_number}"
      pdf.text "Date: #{@order.created_at.strftime('%B %d, %Y')}"
      pdf.text "Due Date: #{@order.created_at.strftime('%B %d, %Y')}" # Same day for events
      pdf.move_down 20

      # Customer details
      pdf.text "Bill To:", style: :bold
      pdf.text @order.user.email
      pdf.move_down 20

      # Order details table
      pdf.text "Event: #{@order.event.name}", style: :bold, size: 14
      pdf.move_down 10

      # Line items
      items = [["Description", "Quantity", "Unit Price", "Total"]]
      
      @order.tickets.group_by(&:ticket_type).each do |ticket_type, tickets|
        items << [
          "#{ticket_type.name} - #{@order.event.name}",
          tickets.count.to_s,
          "€#{ticket_type.price_cents / 100.0}",
          "€#{(tickets.count * ticket_type.price_cents) / 100.0}"
        ]
      end

      pdf.table(items, header: true, width: pdf.bounds.width) do
        row(0).font_style = :bold
        columns(1..3).align = :right
      end

      pdf.move_down 20

      # Total
      pdf.text "Total: €#{@order.total_amount_cents / 100.0}", 
               style: :bold, size: 16, align: :right

      # Footer
      pdf.move_down 40
      pdf.text "Thank you for your purchase!", align: :center
      pdf.text "Generated on #{Time.current.strftime('%B %d, %Y at %I:%M %p')}", 
               align: :center, size: 8
    end.render
  end
end
```

### 2. HTML-to-PDF Solutions

**Using WickedPdf**:
```ruby
# Gemfile
gem 'wicked_pdf'

# app/controllers/invoices_controller.rb
def show
  @order = current_user.orders.find(params[:order_id])
  
  respond_to do |format|
    format.html
    format.pdf do
      render pdf: "invoice_#{@order.id}",
             template: 'invoices/show.html.erb',
             layout: 'pdf'
    end
  end
end
```

### 3. Third-Party Services

**Stripe Invoicing Integration**:
```ruby
# app/services/stripe_invoice_service.rb
class StripeInvoiceService
  def initialize(order)
    @order = order
  end

  def create_post_payment_invoice
    customer = find_or_create_stripe_customer
    
    invoice = Stripe::Invoice.create({
      customer: customer.id,
      collection_method: 'charge_automatically',
      paid: true, # Already paid via checkout
      metadata: {
        order_id: @order.id,
        user_id: @order.user.id
      }
    })

    # Add line items
    @order.tickets.group_by(&:ticket_type).each do |ticket_type, tickets|
      Stripe::InvoiceItem.create({
        customer: customer.id,
        invoice: invoice.id,
        amount: ticket_type.price_cents * tickets.count,
        currency: 'eur',
        description: "#{@order.event.name} - #{ticket_type.name} (#{tickets.count}x)"
      })
    end

    invoice.finalize_invoice
  end

  private

  def find_or_create_stripe_customer
    if @order.user.stripe_customer_id.present?
      Stripe::Customer.retrieve(@order.user.stripe_customer_id)
    else
      customer = Stripe::Customer.create({
        email: @order.user.email,
        metadata: { user_id: @order.user.id }
      })
      
      @order.user.update(stripe_customer_id: customer.id)
      customer
    end
  end
end
```

## Stripe Payment Methods Comparison

### Payment Intents vs Invoicing vs Checkout Sessions

| Feature | Payment Intents | Stripe Invoicing | Checkout Sessions (Current) |
|---------|----------------|------------------|---------------------------|
| **Timing** | Immediate | Deferred (days/weeks) | Immediate |
| **User Experience** | Custom UI on your site | Stripe-hosted invoice page | Stripe-hosted checkout |
| **Payment Methods** | Cards, wallets, BNPL | Cards, bank transfers, checks | Cards, wallets, BNPL |
| **Documentation** | Custom receipts | Formal invoices | Stripe receipts |
| **Integration Complexity** | Medium | Low | Low |
| **Best For** | Custom checkout flows | B2B billing | Quick implementation |

### For Event Ticketing Use Case

**Current Checkout Sessions are ideal** because:
- Events require immediate payment confirmation
- Time-sensitive inventory management
- Users expect instant ticket delivery
- Built-in fraud protection

**Recommended**: Keep Checkout Sessions, add invoice generation for accounting records

## Recommended Implementation

### Phase 1: Database Schema

```ruby
# Migration
class CreateInvoices < ActiveRecord::Migration[8.0]
  def change
    create_table :invoices do |t|
      t.references :order, null: false, foreign_key: true
      t.string :invoice_number, null: false
      t.integer :total_amount_cents, null: false
      t.string :currency, default: 'eur'
      t.string :status, default: 'issued' # issued, paid, cancelled
      t.datetime :issued_at
      t.datetime :paid_at
      t.string :stripe_invoice_id # Optional: if using Stripe
      t.text :notes
      
      t.timestamps
    end
    
    add_index :invoices, :invoice_number, unique: true
    add_index :invoices, :status
  end
end

# Add to User model for Stripe integration
class AddStripeFieldsToUsers < ActiveRecord::Migration[8.0]
  def change
    add_column :users, :stripe_customer_id, :string
    add_index :users, :stripe_customer_id
  end
end
```

### Phase 2: Models

```ruby
# app/models/invoice.rb
class Invoice < ApplicationRecord
  belongs_to :order
  has_one :user, through: :order
  has_one :event, through: :order

  validates :invoice_number, presence: true, uniqueness: true
  validates :total_amount_cents, presence: true, numericality: { greater_than: 0 }
  validates :status, inclusion: { in: %w[issued paid cancelled] }

  before_validation :generate_invoice_number, on: :create
  before_validation :set_defaults, on: :create

  scope :paid, -> { where(status: 'paid') }
  scope :unpaid, -> { where(status: 'issued') }

  def total_amount_euros
    total_amount_cents / 100.0
  end

  def generate_pdf
    InvoicePdfGenerator.new(self).generate
  end

  def mark_as_paid!
    update!(status: 'paid', paid_at: Time.current)
  end

  private

  def generate_invoice_number
    return if invoice_number.present?
    
    year = Time.current.year
    month = Time.current.strftime('%m')
    
    # Find highest invoice number for current month
    last_invoice = Invoice.where(
      'invoice_number LIKE ?', 
      "INV-#{year}#{month}-%"
    ).order(:invoice_number).last
    
    if last_invoice
      sequence = last_invoice.invoice_number.split('-').last.to_i + 1
    else
      sequence = 1
    end
    
    self.invoice_number = "INV-#{year}#{month}-#{sequence.to_s.rjust(4, '0')}"
  end

  def set_defaults
    self.total_amount_cents = order.total_amount_cents if order
    self.issued_at = Time.current
    self.status = 'paid' if order&.status == 'paid' # Auto-mark as paid for completed orders
  end
end

# app/models/order.rb - Add invoice association
class Order < ApplicationRecord
  # ... existing code ...
  has_one :invoice, dependent: :destroy

  # Add method to create invoice after payment
  def create_invoice!
    return invoice if invoice.present?
    
    Invoice.create!(
      order: self,
      total_amount_cents: self.total_amount_cents
    )
  end
end
```

### Phase 3: Controllers

```ruby
# app/controllers/invoices_controller.rb
class InvoicesController < ApplicationController
  before_action :authenticate_user!
  before_action :set_invoice

  def show
    # HTML view of invoice
  end

  def download_pdf
    pdf = @invoice.generate_pdf
    
    send_data pdf,
              filename: "invoice_#{@invoice.invoice_number}.pdf",
              type: 'application/pdf',
              disposition: 'attachment'
  end

  private

  def set_invoice
    @invoice = current_user.invoices.joins(:order).find(params[:id])
  rescue ActiveRecord::RecordNotFound
    redirect_to dashboard_path, alert: "Invoice not found"
  end
end

# Update app/controllers/orders_controller.rb
class OrdersController < ApplicationController
  # ... existing code ...

  def payment_success
    # ... existing payment success logic ...
    
    if stripe_session.payment_status == "paid"
      @order.mark_as_paid!
      
      # Generate invoice
      @order.create_invoice!
      
      # Send confirmation emails with invoice attached
      @order.tickets.each do |ticket|
        begin
          TicketMailer.purchase_confirmation(ticket).deliver_now
        rescue => e
          Rails.logger.error "Failed to send confirmation: #{e.message}"
        end
      end
      
      # ... rest of existing code ...
    end
  end
end
```

### Phase 4: Mailer Updates

```ruby
# app/mailers/ticket_mailer.rb - Update to include invoice
class TicketMailer < ApplicationMailer
  def purchase_confirmation(ticket)
    @ticket = ticket
    @order = ticket.order
    @user = ticket.user
    @event = ticket.event

    # Attach ticket PDF
    ticket_pdf = ticket.to_pdf
    attachments["ticket_#{ticket.id}.pdf"] = ticket_pdf

    # Attach invoice PDF
    if @order.invoice.present?
      invoice_pdf = @order.invoice.generate_pdf
      attachments["invoice_#{@order.invoice.invoice_number}.pdf"] = invoice_pdf
    end

    mail(
      to: @user.email,
      subject: "Your tickets for #{@event.name}"
    )
  end
end
```

### Phase 5: Routes

```ruby
# config/routes.rb
Rails.application.routes.draw do
  # ... existing routes ...
  
  resources :invoices, only: [:show] do
    member do
      get :download_pdf
    end
  end
  
  resources :orders do
    member do
      get :invoice # Shortcut to order's invoice
    end
  end
end
```

### Phase 6: Views

```erb
<!-- app/views/invoices/show.html.erb -->
<div class="max-w-4xl mx-auto p-6">
  <div class="bg-white shadow-lg rounded-lg p-8">
    <div class="flex justify-between items-start mb-8">
      <div>
        <h1 class="text-3xl font-bold text-gray-900">Invoice</h1>
        <p class="text-gray-600">Invoice #<%= @invoice.invoice_number %></p>
      </div>
      <div class="text-right">
        <p class="text-sm text-gray-600">Date: <%= @invoice.issued_at.strftime('%B %d, %Y') %></p>
        <span class="inline-flex px-2 py-1 text-xs font-semibold rounded-full 
                     <%= @invoice.status == 'paid' ? 'bg-green-100 text-green-800' : 'bg-yellow-100 text-yellow-800' %>">
          <%= @invoice.status.capitalize %>
        </span>
      </div>
    </div>

    <!-- Invoice details -->
    <div class="grid grid-cols-2 gap-8 mb-8">
      <div>
        <h3 class="font-semibold text-gray-900 mb-2">Bill To:</h3>
        <p class="text-gray-600"><%= @invoice.user.email %></p>
      </div>
      <div>
        <h3 class="font-semibold text-gray-900 mb-2">Event:</h3>
        <p class="text-gray-600"><%= @invoice.event.name %></p>
        <p class="text-sm text-gray-500"><%= @invoice.event.start_time.strftime('%B %d, %Y at %I:%M %p') %></p>
      </div>
    </div>

    <!-- Line items -->
    <div class="border-t border-b border-gray-200 py-4 mb-8">
      <table class="w-full">
        <thead>
          <tr class="text-left text-gray-500 text-sm">
            <th class="pb-2">Description</th>
            <th class="pb-2 text-right">Qty</th>
            <th class="pb-2 text-right">Unit Price</th>
            <th class="pb-2 text-right">Total</th>
          </tr>
        </thead>
        <tbody>
          <% @invoice.order.tickets.group_by(&:ticket_type).each do |ticket_type, tickets| %>
            <tr>
              <td class="py-2"><%= ticket_type.name %></td>
              <td class="py-2 text-right"><%= tickets.count %></td>
              <td class="py-2 text-right">€<%= ticket_type.price_cents / 100.0 %></td>
              <td class="py-2 text-right">€<%= (tickets.count * ticket_type.price_cents) / 100.0 %></td>
            </tr>
          <% end %>
        </tbody>
      </table>
    </div>

    <!-- Total -->
    <div class="text-right mb-8">
      <p class="text-2xl font-bold text-gray-900">
        Total: €<%= @invoice.total_amount_euros %>
      </p>
    </div>

    <!-- Actions -->
    <div class="flex justify-end space-x-4">
      <%= link_to "Download PDF", download_pdf_invoice_path(@invoice), 
                  class: "bg-blue-600 text-white px-4 py-2 rounded hover:bg-blue-700" %>
    </div>
  </div>
</div>
```

## Integration Guidelines

### 1. Testing Strategy

```ruby
# test/services/invoice_pdf_generator_test.rb
require 'test_helper'

class InvoicePdfGeneratorTest < ActiveSupport::TestCase
  setup do
    @order = orders(:paid_order)
    @generator = InvoicePdfGenerator.new(@order.invoice)
  end

  test "generates PDF successfully" do
    pdf = @generator.generate
    assert pdf.present?
    assert pdf.is_a?(String)
  end

  test "includes order details in PDF" do
    # Test PDF content includes expected information
  end
end
```

### 2. Performance Considerations

```ruby
# app/jobs/invoice_generation_job.rb
class InvoiceGenerationJob < ApplicationJob
  queue_as :default

  def perform(order_id)
    order = Order.find(order_id)
    invoice = order.create_invoice!
    
    # Generate PDF in background
    pdf = invoice.generate_pdf
    
    # Store in cloud storage if needed
    # S3Service.store_invoice_pdf(invoice, pdf)
    
    # Send email notification
    InvoiceMailer.invoice_ready(invoice).deliver_now
  end
end

# Call from OrdersController#payment_success
InvoiceGenerationJob.perform_later(@order.id)
```

### 3. Security Considerations

- Ensure users can only access their own invoices
- Validate invoice numbers are unique
- Sanitize user input in PDF generation
- Use HTTPS for invoice downloads
- Consider adding invoice access tokens for sharing

### 4. Deployment Checklist

- [ ] Run database migrations
- [ ] Update mailer templates
- [ ] Test PDF generation in production environment
- [ ] Verify email attachments work correctly
- [ ] Set up monitoring for PDF generation failures
- [ ] Add invoice generation to payment success flow

## Conclusion

The recommended approach combines the best of both worlds:
- Keep the existing immediate payment flow for better user experience
- Generate professional invoices for accounting and user records
- Use existing Prawn infrastructure for consistent PDF styling
- Optionally integrate with Stripe invoicing for advanced features

This implementation provides a solid foundation that can be extended with additional features like tax calculations, discounts, or integration with accounting systems.