Purchase order PDFs in Django

Why purchase orders become PDFs

A purchase order is the document a supplier's accounts team matches an invoice against, and it travels by email between two organisations that share no system. That is the difference between a document and a view of a database, and it is why procurement teams keep asking for this.

What a purchase order has to carry

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

• the PO number, which the supplier invoice must quote back
• the ordering entity and the ship-to address, which are often different
• the bill-to address and any cost centre or GL code
• each line with the agreed price, not the supplier list price
• the delivery date required, and the approval reference

Turning your the Django template language template into a document

You already have the markup. Render_to_string("invoices/detail.html", context) gives you it as a string, which is the only input the render needs. Django resolves {% static %} to a path relative to STATIC_URL, which means the stylesheet URL in your rendered HTML is only meaningful to a browser that already has a session on your host. A renderer fetching it from outside gets a 404 and produces an unstyled document. Inline the CSS into the string you send, or make STATIC_URL absolute for this one code path.

The Django implementation

httpx is the client most projects here already have. The render happens in a view that returns FileResponse, and you hand the bytes back as a FileResponse with an explicit content type.

# views.py
import httpx
from django.conf import settings
from django.http import FileResponse
from django.shortcuts import get_object_or_404
from django.template.loader import render_to_string

from .models import PurchaseOrder


def purchase_order_pdf(request, pk):
    purchase_order = get_object_or_404(PurchaseOrder, pk=pk)

    # A template that extends nothing. The on-screen template carries base.html,
    # the nav and the messages block, none of which belong in a purchase order.
    html = render_to_string("purchase_orders/pdf.html", {"purchase_order": purchase_order})

    upstream = httpx.stream(
        "POST",
        "https://api.pdfpipe.xyz/v1/pdf",
        headers={"Authorization": f"Bearer {settings.PDFPIPE_KEY}"},
        json={"html": html, "options": {"format": "A4", "printBackground": True}},
        timeout=60.0,
    )

    response = upstream.__enter__()
    response.raise_for_status()

    return FileResponse(
        response.iter_bytes(),
        content_type="application/pdf",
        filename=f"purchase-order-{purchase_order.reference}.pdf",
    )

The CSS that makes a purchase order page correctly

The layout problem specific to this document is that delivery and billing addresses sit alongside a line item table, and both must stay on the first page where a supplier looks for them. These rules handle it.

/* Backgrounds are dropped unless printBackground is set on the
   request, which is the usual reason a document renders in plain
   black and white when it looked right in the browser. */
@page {
  size: A4;
  margin: 18mm 16mm;
}

table { break-inside: avoid; }
h2    { break-after: avoid; }

What goes wrong in Django

The template that renders your on-screen view is almost always the wrong one to send. It carries the base layout, the nav, and the messages framework, all of which land in the PDF. Render a template that extends nothing, and keep it beside the view template so the two do not drift.

What people try first

Most Django projects reach for WeasyPrint or xhtml2pdf, both of which put layout in your Python process. WeasyPrint needs Cairo and Pango in the image; xhtml2pdf supports a subset of CSS that stopped growing years ago. 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 purchase order lives afterwards

Rendering is the short part. A PO is retained alongside the invoice it authorised, because the pair is what an auditor matches. In practice it lives in the procurement system for as long as the supplier relationship does, which is longer than most teams plan for.

Getting the document right

• Check the PO number, because an invoice quoting the wrong one will not be paid.
• Generation is triggered by an approval completing in a procurement workflow, so size the timeout for that path rather than for a health check.
• These are produced one at a time by a person who is waiting, so latency is felt directly.
• 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 Django documents, and the same purchase order in other stacks.