The legs must sum to zero

That is the whole of double entry, and it is checked before anything is written. £81.30 on the card, £61.30 to food, £25.00 to travel, £5.00 back in cash — four legs, one entry, one transaction.

⛔ All of it lands or none of it does. The entry and its legs go through core.records.batch, so there is no moment where the entry exists and its legs do not. Written one at a time, a crash halfway leaves a half-posted entry that looks posted and does not balance — and nothing downstream can tell that from an entry that never balanced in the first place.

Why the check is in pennies

⛔ Adding the amounts as decimals would refuse valid entries. -100.10 + 30.20 + 69.90 is 0.0000000000000142 in floating point, not zero. So every amount is multiplied by 100 and rounded to a whole penny first, and the sum of those integers is what has to be zero. Integer arithmetic is exact well past any amount you will ever type.

⚠ More than two decimal places is refused, not rounded. Otherwise the check would pass on the rounded figure while the stored amount said something else — a balanced entry made of unbalanced numbers. If you need thirds of a penny, decide where the odd one goes before you post.

⚠ The refusal has a tolerance, and it has to. 69.90 × 100 is 6990.000000000001, so an exact test would reject a perfectly ordinary amount. A genuine third decimal is out by at least 0.0999; float noise is out by around 1e-12. The line sits between them, four orders of magnitude clear of both.

Ids, and posting the same thing twice

⚠ Every leg needs its own id, and the entry's ref is its id. That is what makes re-posting safe: create is replay-safe, so sending the identical entry again writes nothing rather than duplicating it. Without stable leg ids a retry would replay the entry and duplicate its legs, which is the worst of both.

⛔ A ref already used by a DIFFERENT entry is refused — same id, different content, so the write conflicts. That is a uniqueness check you get for free rather than one this recipe has to remember to do.

One currency per entry

⛔ The currency comes from each leg's ACCOUNT, and every leg must agree. It is not something you type — a typed currency is a second source of truth that can quietly disagree with the account the money actually sits in.

⛔ All legs must be in one currency. -100 GBP and +100 USD sums to zero and balances nothing: a zero-sum check across currencies is arithmetic coincidence, not an invariant, and it would wave through an entry that is wrong in both books. A currency exchange is TWO entries through a clearing account — one in each currency — which is what double entry does anyway, and it leaves the rate visible as the difference between them rather than buried in a conversion.

What it does not do

⚠ It does not tag anything. Legs carry tags through leg_tag, which is a separate step — an entry is about money moving between accounts, and what you call it afterwards is a different decision.