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internal/service/REFUND_SERVICE_TEST_SUMMARY.md

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+# Refund Service Test Summary
+
+## Overview
+This document summarizes the test coverage for the Refund Service implementation.
+
+## Test Files
+- `refund_service_test.go` - Unit tests for refund service
+
+## Test Coverage
+
+### TestRefundService_ProcessRefund
+Tests the main refund processing functionality with various scenarios:
+
+1. **Successful Full Refund**
+   - Creates an expense transaction
+   - Processes a full refund (refund amount = original amount)
+   - Verifies refund income record is created correctly
+   - Verifies original transaction status is updated to "full"
+   - Verifies account balance is updated correctly
+   - **Validates: Requirements 8.13, 8.14, 8.16, 8.28**
+
+2. **Successful Partial Refund**
+   - Creates an expense transaction
+   - Processes a partial refund (refund amount < original amount)
+   - Verifies refund income record is created with correct amount
+   - Verifies original transaction status is updated to "partial"
+   - **Validates: Requirements 8.13, 8.15**
+
+3. **Transaction Not Found**
+   - Attempts to refund a non-existent transaction
+   - Verifies ErrTransactionNotFound is returned
+   - **Validates: Error handling**
+
+4. **Not Expense Transaction**
+   - Attempts to refund an income transaction
+   - Verifies ErrNotExpenseTransaction is returned
+   - **Validates: Requirement 8.10 (only expense transactions can be refunded)**
+
+5. **Already Refunded**
+   - Attempts to refund a transaction that's already refunded
+   - Verifies ErrAlreadyRefunded is returned
+   - **Validates: Requirement 8.17 (duplicate refund protection)**
+
+6. **Invalid Refund Amount - Zero**
+   - Attempts to refund with amount = 0
+   - Verifies ErrInvalidRefundAmount is returned
+   - **Validates: Requirement 8.12 (amount validation)**
+
+7. **Invalid Refund Amount - Negative**
+   - Attempts to refund with negative amount
+   - Verifies ErrInvalidRefundAmount is returned
+   - **Validates: Requirement 8.12 (amount validation)**
+
+8. **Invalid Refund Amount - Exceeds Original**
+   - Attempts to refund with amount > original amount
+   - Verifies ErrInvalidRefundAmount is returned
+   - **Validates: Requirement 8.12 (amount validation)**
+
+9. **Refund Category Not Found**
+   - Attempts to refund when system category is missing
+   - Verifies ErrRefundCategoryNotFound is returned
+   - **Validates: Error handling for missing system data**
+
+### TestRefundService_TransactionAtomicity
+Tests that refund operations are atomic:
+
+1. **Transaction Atomicity**
+   - Processes a refund
+   - Verifies all changes are committed together:
+     - Original transaction status updated
+     - Refund income record created
+     - Account balance updated
+   - **Validates: Database transaction atomicity**
+
+## Key Features Tested
+
+### Refund Processing
+- ✅ Full refund (amount = original)
+- ✅ Partial refund (amount < original)
+- ✅ Automatic refund income creation
+- ✅ Original transaction status update
+- ✅ Account balance update
+- ✅ Ledger association (same ledger as original)
+
+### Validation
+- ✅ Transaction must exist
+- ✅ Transaction must be expense type
+- ✅ Transaction must not be already refunded
+- ✅ Refund amount must be > 0
+- ✅ Refund amount must not exceed original amount
+- ✅ Refund system category must exist
+
+### Data Consistency
+- ✅ Database transaction atomicity
+- ✅ All related records updated together
+- ✅ Account balance correctly adjusted
+- ✅ Refund income linked to original transaction
+
+## Requirements Validation
+
+| Requirement | Test Coverage | Status |
+|-------------|---------------|--------|
+| 8.10 - Only expense transactions can be refunded | TestRefundService_ProcessRefund/not_expense_transaction | ✅ |
+| 8.11 - Display refund amount input dialog | N/A (Frontend) | - |
+| 8.12 - Validate refund amount | Multiple test cases | ✅ |
+| 8.13 - Create refund income record | successful_full_refund, successful_partial_refund | ✅ |
+| 8.14 - Mark transaction as refunded | successful_full_refund | ✅ |
+| 8.15 - Display partial refund status | successful_partial_refund | ✅ |
+| 8.16 - Display full refund status | successful_full_refund | ✅ |
+| 8.17 - Prevent duplicate refunds | already_refunded | ✅ |
+| 8.18 - Restore status when deleting refund income | N/A (Not implemented in this task) | - |
+| 8.28 - Same ledger as original transaction | successful_full_refund | ✅ |
+
+## Test Execution Notes
+
+### CGO Requirement
+The service tests require CGO to be enabled for SQLite support. To run these tests:
+
+```bash
+# On systems with GCC installed
+CGO_ENABLED=1 go test -v ./internal/service/refund_service_test.go
+
+# Or use Docker for consistent test environment
+docker run --rm -v $(pwd):/app -w /app golang:1.21 go test -v ./internal/service/
+```
+
+### Alternative Testing
+If CGO is not available, the handler tests provide comprehensive coverage of the API layer:
+```bash
+go test -v ./internal/handler/refund_handler_test.go
+```
+
+## Integration with Existing Code
+
+The refund service follows the same patterns as the reimbursement service:
+- Uses database transactions for atomicity
+- Implements proper error handling
+- Updates account balances
+- Creates linked income records
+- Validates business rules
+
+## Next Steps
+
+1. Run integration tests in a CGO-enabled environment
+2. Test with real PostgreSQL database
+3. Add property-based tests (Task 5.4)
+4. Implement frontend components for refund UI