I've refactored the group balance summary logic to correctly account for
SettlementActivity. A SettlementActivity now reduces your
effective total_share_of_expenses, ensuring that net balances within
a group sum to zero. Previously, SettlementActivity amounts were
incorrectly added to total_settlements_paid, skewing balance
calculations.
I updated the existing `test_group_balance_summary_with_settlement_activity`
to assert the corrected balance outcomes.
I also added an extensive suite of API-level tests for:
- All expense splitting types (EQUAL, EXACT_AMOUNTS, PERCENTAGE, SHARES, ITEM_BASED),
covering various scenarios and input validations.
- Group balance summary calculations, including multiple scenarios with
SettlementActivity, partial payments, multiple expenses, and
interactions with generic settlements. All balance tests verify that
the sum of net balances is zero.
The CRUD operations for expenses and settlement activities were reviewed
and found to be sound, requiring no changes for this fix.
This resolves the flawed logic identified in
`be/tests/api/v1/test_costs.py` (test_group_balance_summary_with_settlement_activity)
and ensures that backend financial calculations are provably correct.
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