Ticket PDFs in Ruby on Rails

Why tickets become PDFs

A ticket has to work at a gate with no signal, on a phone that may be offline, or on paper. Anything requiring a live request fails at exactly the wrong moment. That is the difference between a document and a view of a database, and it is why event organisers and travel operators keep asking for this.

What a ticket has to carry

Before any of the code below matters, the template has to produce a document that is actually a ticket. These are the fields that make it one, and the ones a reader or an auditor will look for first.

• the scannable code, which is the ticket; everything else is decoration
• the event or journey, with date and time including timezone
• the seat, zone or class where the entitlement is specific
• the holder name where the ticket is non-transferable
• the terms of entry, which are what a gate refusal is justified by

Turning your ERB template into a document

You already have the markup. Render_to_string(template: "invoices/pdf", layout: false) gives you it as a string, which is the only input the render needs. stylesheet_link_tag emits a digested path like /assets/application-9a2f.css, which only exists on your host. For a renderer that does not share your origin, either inline the stylesheet with Rails.application.assets or point at an absolute URL on a public CDN.

The Ruby on Rails implementation

Net::HTTP ships with the runtime, so this adds no dependency. The render happens in a controller action using send_data, and you send_data with type: "application/pdf".

# app/controllers/tickets_controller.rb
require "net/http"

class TicketsController < ApplicationController
  def pdf
    ticket = Ticket.find(params[:id])

    # layout: false is not optional. Without it the application layout wraps the
    # document and your site navigation prints across the top of page one.
    html = render_to_string(
      template: "tickets/pdf",
      layout: false,
      assigns: { ticket: ticket },
    )

    uri = URI("https://api.pdfpipe.xyz/v1/pdf")
    request = Net::HTTP::Post.new(uri)
    request["Authorization"] = "Bearer #{Rails.application.credentials.pdfpipe_key}"
    request["Content-Type"] = "application/json"
    request.body = {
      html: html,
      options: { format: "A4", printBackground: true },
    }.to_json

    upstream = Net::HTTP.start(uri.host, uri.port, use_ssl: true, read_timeout: 60) do |http|
      http.request(request)
    end

    # ASCII-8BIT, and it must stay that way. Anything that assumes UTF-8 here
    # corrupts the file with no error to explain it.
    send_data upstream.body,
              filename: "ticket-#{ticket.reference}.pdf",
              type: "application/pdf",
              disposition: "attachment"
  end
end

The CSS that makes a ticket page correctly

The layout problem specific to this document is that a barcode or QR code must render at a size and contrast a scanner can actually read after printing. These rules handle it.

/* The code has to survive printing. Keep it black on white at a
   size a scanner can resolve, and never let it break across a page. */
.barcode {
  break-inside: avoid;
  image-rendering: pixelated;
  min-width: 45mm;
  filter: none;
}

@page { size: A6 landscape; margin: 5mm; }

What goes wrong in Ruby on Rails

Forgetting layout: false is the single most common mistake here. Without it the application layout wraps the document, and the PDF arrives with your site navigation printed across the top of page one.

What people try first

Most Ruby on Rails projects reach for wicked_pdf, which shells out to wkhtmltopdf, or Prawn, which is excellent but means describing the document in Ruby drawing calls rather than in HTML you already have. That works until it is running on more than one machine, at which point the browser becomes the thing you operate rather than the thing you use.

Where the ticket lives afterwards

Rendering is the short part. A ticket is worthless the moment the event ends, so nothing needs to survive it except the record that it was scanned. That record lives in your database, not in the PDF.

Getting the document right

• Check the code itself. A ticket that will not scan is worthless no matter how it looks.
• Generation is triggered by a booking being confirmed, so size the timeout for that path rather than for a health check.
• Volume arrives in bursts, so queue the render rather than doing it inside the request that triggered it.
• Backgrounds are painted by default here, so a design that uses colour needs nothing set. Only an explicit print_background of false turns them off.

Frequently asked

Related

Other Ruby on Rails documents, and the same ticket in other stacks.