Make illegal states unrepresentable: procurement as a state machine

The scariest bugs in an ERP aren't crashes. They're silent ones: a purchase order edited after approval, a delivery recorded against a cancelled PO. Nothing throws. The books are just wrong.

Booleans are how you get hurt

The naive model is a row with flags: isApproved, isDelivered, isCancelled. Eight combinations, most of them meaningless. isDelivered && isCancelled — what does that even mean? Your code has to defend against it everywhere, and one missed check corrupts financial data.

One column, explicit transitions

type POState = "draft" | "submitted" | "approved" | "delivered" | "closed" | "cancelled";

const transitions: Record<POState, POState[]> = {
  draft:     ["submitted", "cancelled"],
  submitted: ["approved", "cancelled"],
  approved:  ["delivered", "cancelled"],
  delivered: ["closed"],
  closed:    [],
  cancelled: [],
};

Every mutation goes through one function that checks the transition table, writes an immutable audit row, and applies role rules (a site engineer can submit an MRN; only an admin approves the PO — and never their own).

What this bought us

Edits after approval became impossible by construction, not by discipline. The audit trail became the source of truth for reconciliation. And when finance asked "who changed this and when?", the answer was one query, not an investigation.

State machines aren't academic. They're the cheapest insurance a backend can buy.