Stop storing what you can derive

The moment you store a status that is a function of other data, you have two sources of truth that can disagree. Every code path that touches a payment now has to remember to recompute the status. One that forgets creates a record that lies.

// Derived at read time — cannot drift, because there is nothing to keep in sync.
function displayStatus(invoice) {
  if (invoice.status === "cancelled") return "cancelled";
  if (invoice.amountPaid >= invoice.total) return "fully_paid";
  if (invoice.amountPaid > 0) return "partially_paid";
  if (isPast(invoice.dueDate)) return "overdue";
  return "not_paid";
}

Facts and decisions. That a payment of ₹50,000 was received on the 3rd is a fact. That a user cancelled the invoice is a decision. Neither can be recomputed from anything else, so both must be stored.

Whether the invoice is overdue is neither — it is a conclusion from the due date and the payments. Store it and you have created a value that goes stale at midnight.

Project progress should come from completed tasks, not a percentage someone types. Stock on hand should come from the movement ledger, not a counter that drifts. In both cases the derived version is slightly more expensive to read and enormously cheaper to trust.

Performance, and only with evidence. If a report genuinely cannot be computed fast enough, cache it — but treat the cache as a cache: give it a clear invalidation path and keep the derivation as the source of truth. That is very different from storing the value as if it were a fact.