example_service_object.rb

# This is a Service Object for generating special PDF with a list of countries. 

# country_pdf_list.rb

# I prefer to use Sendi Matz rule

# Classes can be no longer than one hundred lines of code.
# Methods can be no longer than five lines of code.
# Pass no more than four parameters into a method. Hash options are parameters.
# Controllers can instantiate only one object

require "wicked_pdf"

class CountryPdfList
  def call
    save_data_to_pdf(pdf_infection_country_render)
  end

  def file_path
    Rails.root.join('tmp', 'pdfs', "pdf_infection_country_list_#{time_now}.pdf")
  end

  private

  def pdf_infection_country_render
    ac = ActionController::Base.new.render_to_string(pdf_params)
    WickedPdf.new.pdf_from_string(ac) 
  end

  def pdf_params
    {
      template: "admin/travel_check/countries/generate_list_of_country_pdf.html.erb",
      title: "Countries Infection List, #{time_now}",
      encoding: 'utf-8' 
    }
  end
  
  def save_data_to_pdf(pdf_file)
    File.open(file_path, 'wb') do |file|
      file << pdf_file
    end
  end

  def time_now
    Time.now.strftime("%Y-%m-%d")
  end
end


# method from controller.rb

def generate_list_of_country_pdf
  create_pdf_list = CountryPdfList.new
  create_pdf_list.call

  send_file create_pdf_list.file_path, :disposition => 'attachment'
end


# Also it eaysy to test 

require 'rails_helper'

RSpec.describe CountryPdfList, type: :service do

  before(:each) do
    @pdf_country_list = CountryPdfList.new.create_country_pdf
  end
  
  context "create a pdf with countrry list" do
    it { @pdf_country_list.create_country_pdf be_kind_of(File) }

    it "create_country_pdf " do
      expect(@pdf_country_list).with("pdf_infection_country_list_2020-02-07.pdf", "w")

      @pdf_country_list.should be_true

      File.should_receive(:open).with("filename", "w").and_yield(file)
    end 

    it 'generated PDF file' do      
      filename = "pdf_infection_country_list_2020-02-07.pdf"
      allow(File).to receive(:open).with(filename,'w')
    
      # call the function that writes to the file
      File.open(@filename, 'w') {|f| f.write(@content)}
    end
  end
end