## Why
Some accounts are shared. A joint account funded 50/50 with a partner
holds money that is only half yours, so its expenses and income should
only count towards your figures by half — otherwise the dashboard and
the cashflow screen overstate both sides of every month.
## What
An account can now be configured with **the share of it you actually
own**, in Settings → Accounts → Edit account.
- **Transactions are always weighted** by that percentage. This covers
the cashflow screen (summary, sankey, trend, category breakdown) and the
dashboard (cashflow summary, monthly spending, top categories).
- **Balances stay at face value by default**, so an account keeps
matching what the bank shows. An opt-in checkbox — only offered below
100% — applies the same percentage to the balance as well, which then
flows through net worth, its evolution, and the account cards.
Defaults are 100% / opt-in off, so existing users see no change at all.
## How
The share is computed at exactly two choke points, one per code path:
- **PHP** — `ConvertsTransactionCurrency::convertTransactionAmount()`
applies `Account::shareOfAmount()` after the currency conversion,
covering every row-by-row analytics consumer.
- **SQL** — `Transaction::OWNED_AMOUNT_SQL` plus the
`joinOwningAccount()` scope weigh the four aggregate queries that never
hydrate models.
Balances go through `BalanceLookup::forAccounts()`, the single point
every net-worth, evolution and account-metrics reader already shares —
so they all stay consistent for free, while the surfaces that must show
the real bank figure (the balance editor, imports, bank sync) read
`account_balances` directly and are untouched.
`ownership_percentage` is a **signed** `tinyInteger` on purpose: MySQL
promotes `signed * unsigned` to `BIGINT UNSIGNED`, which overflows on
the negative amount of an expense. A test caught this.
## Deliberate boundaries
- **Transaction rows keep showing the real amount.** A row should say
what the bank actually charged; only totals are your share.
- **The balance editor writes the full amount**, and now says so when
the account is shared — that is what stops a shared balance being halved
again on every correction.
- **Percentages are whole numbers, 1–100.** A three-way split (33.33%)
is not expressible yet.
- **Configurable on edit only.** `StoreAccountRequest` and the create
form are unchanged, so a new joint account counts at 100% until you edit
it.
## Known gap (follow-up)
**Budgets are not weighted.** `budget_transactions.amount` is a snapshot
written at assignment time, so a 50% account still counts at 100% in
budget spend, carry-over and threshold alerts. Fixing it properly needs
the write path weighted, a re-snapshot when the percentage changes, and
a backfill of existing rows — a separate PR rather than a line in this
one. Until then a shared account can read €400 on the dashboard and €800
in Budgets for the same category.
Loan/mortgage *projections* also seed off the raw latest balance, so a
shared **loan** with the balance opt-in on draws a step at today's date.
Narrow, and follow-up material.
## Testing
`tests/Feature/SharedAccountOwnershipTest.php` covers every weighted
endpoint, net worth with and without the balance opt-in, the 1–100
validation, and pins the PHP and SQL rounding to the same answer on an
uneven share (33% of an odd amount).
Manual QA on real local data (June 2026, account "Daily" set to 50%):
| | Before | After |
|---|---|---|
| Cashflow income | €4,460 | €4,057 (−€403 = half of the account's
€805.80) |
| Cashflow expenses | €4,122 | €3,217 (−€905 = half of the account's
€1,809.85) |
| Net worth (opt-in off) | 30,516,453 | 30,516,453 — unchanged |
| Net worth (opt-in on) | 30,516,453 | 30,470,797 (−45,656 = half of the
€913.12 balance) |
Also verified: the balance editor still prefills the real €244,310.08
and shows the shared-account notice; an empty percentage field leaves
the setting untouched instead of resetting to 100%; out-of-range values
are rejected.
## Demo
https://github.com/user-attachments/assets/de47e41a-ff72-4b2b-8e5b-51076e048504
<!-- PLACEHOLDER: drag the QA video here -->
c9a70ffa1a