Use CombinePDF in a Rails app to merge existing PDF files with Prawn generated PDF content

appel_pdf.rb

# app/pdfs/appel_pdf.rb
class AppelPdf < Prawn::Document
  def initialize(appel)
    text 'Content for Appel PDF'
  end
end

document.rb

# app/models/document.rb
class Document < ActiveRecord::Base
  belongs_to :foobar
  
  has_attached_file :attachment
  validates_attachment :attachment,
    content_type: { content_type: ['image/jpeg', 'image/gif', 'image/png', 'application/pdf']
end

foobar.rb

# app/models/foobar.rb
class Foobar < ActiveRecord::Base
  has_many :documents
  
  def pdf_documents
    documents.where('attachment_content_type LIKE ?', '%pdf%').map do |pdf_document|
      File.new(pdf_document.attachment.path)
    end
  end
end

foobars_controller.rb

class FoobarsController < ApplicationController
  def show
    @foobar = Foobar.find(params[:id])
    respond_to do |format|
      format.pdf do
        foobar_pdf = FoobarPdf.new(@foobar).render
        final_pdf = CombinePDF.new
        final_pdf << CombinePDF.parse(foobar_pdf)
        if @foobar.pdf_documents.any?
          @foobar.pdf_documents.each do |pdf_document|
            final_pdf << CombinePDF.parse(IO.read(pdf_document))
          end
        end
        send_data final_pdf.to_pdf, filename: "foobar_#{@foobar.id}", type: 'application/pdf', disposition: 'inline'
      end
    end
  end
end

Gemfile

gem 'combine_pdf'
gem 'prawn'
gem 'paperclip'