Report PDFs in Ruby on Rails

Why reports become PDFs

A report is a snapshot of numbers at a moment in time. Its whole value is that it does not change when the underlying dashboard does. That is the difference between a document and a view of a database, and it is why analytics, operations and account teams keep asking for this.

What a report has to carry

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

• the reporting period as an explicit start and end date
• the generation timestamp, distinct from the period
• the data source or filter set, so a reader can tell what was excluded
• each metric with its unit, since a bare number is not a figure
• comparison against the prior period where the report claims a trend

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/reports_controller.rb
require "net/http"

class ReportsController < ApplicationController
  def pdf
    report = Report.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: "reports/pdf",
      layout: false,
      assigns: { report: report },
    )

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

The CSS that makes a report page correctly

The layout problem specific to this document is that charts and tables must not split mid-element, and a report long enough to matter will page-break somewhere awkward unless break rules are set. These rules handle it.

/* Charts and tables must not split. Sections start on a fresh
   page so a heading never ends up alone at the foot of one. */
.chart, figure, table { break-inside: avoid; }
h2 { break-after: avoid; }
section { break-before: page; }
section:first-of-type { break-before: auto; }

@page { size: A4; margin: 20mm 18mm; }

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 report lives afterwards

Rendering is the short part. Reports pile up faster than any other document here because they are produced on a schedule whether or not anyone reads them. Most teams keep twelve months and expire the rest, which makes storage lifecycle a design decision rather than an afterthought.

Getting the document right

• Check the period the figures cover, because a report without an unambiguous date range is worse than no report.
• Generation is triggered by a schedule, usually month end or week end, so size the timeout for that path rather than for a health check.
• This is a batch shape: render the run through the batch endpoint rather than firing thousands of individual requests.
• 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 report in other stacks.