feat: experiment with MailerPreview

app/mailers/invoice_mailer.rb

@@ -15,6 +15,7 @@ def finalized
     I18n.locale = @customer.preferred_document_locale
 
     @invoice.file.open do |file|
+      pp file
       attachments['invoice.pdf'] = file.read
     end
 

config/environments/development.rb

@@ -63,6 +63,8 @@
     address: 'mailhog',
     port: 1025,
   }
+  config.action_mailer.default_url_options = { host: 'app.lago.dev' }
+  config.action_mailer.preview_path = "#{Rails.root}/spec/mailer_previews"
 
   Dotenv.load
 end

spec/factories/invoices.rb

@@ -12,6 +12,16 @@
 
     organization_sequential_id { rand(1_000_000) }
 
+    trait :with_file do
+      after(:build) do |model|
+        model.file.attach(
+          io: File.open(Rails.root.join('spec/fixtures/blank.pdf')),
+          filename: 'blank.pdf',
+          content_type: 'application/pdf',
+        )
+      end
+    end
+
     trait :draft do
       status { :draft }
     end

spec/mailer_previews/base_preview.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+class BasePreview < ActionMailer::Preview
+  def self.call(...)
+    message = nil
+    ActiveRecord::Base.transaction do
+      message = super(...)
+      raise ActiveRecord::Rollback
+    end
+    message
+  end
+end

spec/mailer_previews/invoice_mailer_preview.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+class InvoiceMailerPreview < BasePreview
+  def finalized
+    invoice = FactoryBot.create(:invoice, :with_file, {
+      fees_amount_cents: 121_49,
+      total_amount_cents: 1000_00,
+    })
+
+    InvoiceMailer.with(invoice:).finalized
+  end
+end