[joaoguimaraespro]

Summary

"Why can't a transaction have 2 categories?" — now it can. One movement (one bank line, one total) can be split into category lines, e.g. one energy bill: electricity 61,20 € + gas 35,20 €.

• Quick-add/edit: Dividir por categorias turns the category field into lines (category + amount + optional note), add/remove lines, live "Faltam X / acima do total / Soma igual ao total", Preencher restante, a nature option Por categoria; refuses to save until the lines add up. Keyboard friendly, fits 390px.
• List: a split row shows "2 categorias" and expands to its lines. Filtered by a category, the row shows its part in that category ("de 96,40 €").
• Recurring items can be split too (form + list); proposals and confirmed transactions carry the lines.

Data model (and why)

• New owned table finance.transaction_splits (id, transaction_id, position, category_id → categories RESTRICT, original_amount, base_amount, nature, note ≤120); cascade-deleted with the transaction, hidden with it by the soft-delete filter.
• A split transaction has category_id = NULL and keeps its total/account/date/description. Alternatives considered: keeping a "main" category on the parent would double-count or need every query to special-case it; a separate "split" pseudo-category leaks into reports. With NULL, any category-grouped query must read the lines, and the parent stays the single movement for balances, interest and audit.
• Rules (domain + request validator): expense/income only; 2–20 lines; amount > 0 (≤4 decimals); each category once and of the transaction's type; lines sum exactly to the amount in the original currency; not with a single category at the same time; never on transfers, savings, investments, broker rows or estimated interest (those are already read-only).
• EUR: each line = round(amount × fx, 4), remainder on the last line, so lines always sum to the transaction's base_amount.
• Nature: the transaction's explicit nature applies to every line; otherwise each expense line takes its category's default nature (Variable if none), stored on the line at write time — same behaviour as single-category rows. Parent nature is NULL for a split unless explicit.
• Audit: line changes are recorded on the transaction (Splits: "60.00 <cat>; 40.00 <cat>" from → to), including edits that only touch lines; re-saving identical lines adds no row.
• Recurring: owned table finance.recurring_transaction_splits; confirming with a different amount rescales lines proportionally (cents, remainder last). The expected list shows the scaled lines.

Where splits are counted

• LedgerAggregates.MonthTotalsAsync (the single source for monthly, annual, overview, trends, category breakdown, budgets/category limits, budget notifications, dashboard): unsplit rows grouped as before + split lines grouped by line category/nature + split transactions counted once.
• Transaction list: category filter matches any line (parent includes children) with categoryAmount; nature filter matches lines.
• Interest: an interest line of a split income counts as real interest (settles the month, shows in account interest).
• Exports: CSV and Excel — one row per line with split ("1/2") and split_note columns appended at the end (columns sum by category to the report figures). JSON archive: splits on transactions and recurring items; import re-derives EUR amounts and skips a record whose line category can't be mapped. Round-trip covered by test.
• AI tools: summaries count lines (get_expense_summary total/count, get_income_summary, get_budget_status, get_transactions totals with a category count only matching lines); rows show splits: [{category, amount}] (get_transactions, get_expense_transactions, get_recurring). create_transaction/update_transaction accept optional splits as "electricity: 60.00; water-gas: 40" — a string so every tool argument stays scalar (the architecture test pinning "no arrays/objects" is unchanged); same validation, strictly parsed. Updating only the amount of a split row is refused (lines must add up); upsert_recurring keeps a split template's lines and refuses changing its amount.

Migration

20261005214742_TransactionSplits — creates the two tables + indexes; no data change, existing rows are unaffected. New tables get app grants through the existing default privileges.

Test plan

• dotnet build, all backend tests (330 passed): new Finance.Tests/SplitTransactionTests (validation, rounding/base amounts, nature, sources, recurring rescale) and Integration.Tests/SplitTransactionTests (create/edit/remove + audit, reports fixed/variable, category budget status, list category/nature filter with categoryAmount, refusals for transfer/investment/sum/duplicates/income category, CSV rows, recurring proposal + confirm, AI read counts and strict writes); JSON round-trip test now includes a split.
• Web build + ng test (113 passed) incl. split-lines.spec.ts.
• Full Docker E2E locally (journey.spec.ts extended with a split expense: split, pick a category, fill remaining, save, expand in list).
• Screenshots (mocked API): quick-add split at 1280 light / 390 dark, list at both.
• Note: AiToolCoverageTests year breakdown moved from 2041 to 2042 — it raced with AiWriteToolsTests (2041 income) and failed depending on order.
• Note: the initial bundle is ~803 kB on main already (over the 800 kB warning); this PR adds ~0.05 kB.