Packing slip PDFs in Ruby on Rails

Why packing slips become PDFs

A packing slip is printed and physically placed in a box. It exists to be paper. That is the difference between a document and a view of a database, and it is why warehouse and fulfilment teams keep asking for this.

What a packing slip has to carry

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

• the order reference, which the picker checks against the pick list
• each line with SKU, description and quantity shipped
• backordered lines shown explicitly rather than omitted
• the ship-to address as it will appear on the carton
• the carrier reference or tracking number where it exists

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/packing_slips_controller.rb
require "net/http"

class PackingSlipsController < ApplicationController
  def pdf
    packing_slip = PackingSlip.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: "packing_slips/pdf",
      layout: false,
      assigns: { packing_slip: packing_slip },
    )

    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: "packing-slip-#{packing_slip.reference}.pdf",
              type: "application/pdf",
              disposition: "attachment"
  end
end

The CSS that makes a packing slip page correctly

The layout problem specific to this document is that it must print correctly on the label and A5 stock warehouses actually use, not just on A4. These rules handle it.

/* Warehouses print on A5 and label stock, not A4. */
@page {
  size: A5;
  margin: 8mm;
}

.signature {
  break-inside: avoid;
  margin-top: auto;
}

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 packing slip lives afterwards

Rendering is the short part. The packing slip is thrown away when the box is opened. It has essentially no retention requirement, which is unusual on this list and worth knowing: the document does not need to be stored, only printed correctly the first time.

Getting the document right

• Check the order reference and quantities, which a picker checks against the physical goods.
• Generation is triggered by an order moving to fulfilment, 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 packing slip in other stacks.