# Transaction Model Extension Implementation

## Overview

This document describes the implementation of Task 1.5: 扩展Transaction模型 (Extend Transaction Model) from the accounting-feature-upgrade specification.

## Implementation Summary

### 1. Model Extensions

Extended the `Transaction` model in `backend/internal/models/models.go` with the following new fields:

#### Multi-Ledger Support
- **LedgerID** (`*uint`): Associates transaction with a specific ledger
- Validates: Requirements 3.10

#### Precise Time Recording
- **TransactionTime** (`*time.Time`): Records precise transaction time (HH:mm:ss)
- Validates: Requirements 5.2

#### Reimbursement Fields
- **ReimbursementStatus** (`string`): Status of reimbursement (none, pending, completed)
- **ReimbursementAmount** (`*float64`): Amount to be reimbursed
- **ReimbursementIncomeID** (`*uint`): Links to the generated reimbursement income transaction
- Validates: Requirements 8.4-8.9

#### Refund Fields
- **RefundStatus** (`string`): Status of refund (none, partial, full)
- **RefundAmount** (`*float64`): Amount refunded
- **RefundIncomeID** (`*uint`): Links to the generated refund income transaction
- Validates: Requirements 8.10-8.18

#### Original Transaction Link
- **OriginalTransactionID** (`*uint`): Links refund/reimbursement income back to original expense
- **IncomeType** (`string`): Type of income (normal, refund, reimbursement)
- Validates: Requirements 8.19-8.22

#### New Relationships
- **Ledger**: Foreign key relationship to Ledger model
- **Images**: One-to-many relationship with TransactionImage
- **OriginalTransaction**: Self-referencing relationship for refund/reimbursement tracking

### 2. Database Migration

Created migration file: `backend/migrations/003_extend_transaction_model.sql`

The migration adds the following columns to the `transactions` table:
- `transaction_time` (TIME)
- `reimbursement_status` (VARCHAR(20), default: 'none')
- `reimbursement_amount` (DECIMAL(15,2))
- `reimbursement_income_id` (BIGINT UNSIGNED)
- `refund_status` (VARCHAR(20), default: 'none')
- `refund_amount` (DECIMAL(15,2))
- `refund_income_id` (BIGINT UNSIGNED)
- `original_transaction_id` (BIGINT UNSIGNED)
- `income_type` (VARCHAR(20))

Indexes created:
- `idx_transactions_reimbursement_income_id`
- `idx_transactions_refund_income_id`
- `idx_transactions_original_transaction_id`

### 3. Test Coverage

Created comprehensive test file: `backend/internal/models/transaction_extension_test.go`

Test functions:
1. **TestTransactionExtensionFields**: Verifies all new fields are properly set
2. **TestTransactionReimbursementStatuses**: Validates reimbursement status values
3. **TestTransactionRefundStatuses**: Validates refund status values
4. **TestTransactionIncomeTypes**: Validates income type values
5. **TestTransactionDefaultValues**: Verifies default values for new fields
6. **TestTransactionRelationships**: Verifies new relationship fields
7. **TestTransactionPreciseTime**: Tests precise time recording functionality
8. **TestTransactionReimbursementFlow**: Tests complete reimbursement workflow
9. **TestTransactionRefundFlow**: Tests complete refund workflow (full and partial)
10. **TestTransactionOriginalLink**: Tests original transaction linking
11. **TestTransactionLedgerAssociation**: Tests ledger association

All tests pass successfully ✓

### 4. Migration Execution

The migration was successfully executed using:
```bash
go run cmd/migrate/main.go
```

Results:
- ✓ All existing tables updated
- ✓ New `transaction_images` table created
- ✓ New `user_settings` table created
- ✓ System categories initialized (refund, reimbursement)

## Field Details

### Reimbursement Status Values
- `none`: No reimbursement requested
- `pending`: Reimbursement requested, awaiting confirmation
- `completed`: Reimbursement confirmed and income record created

### Refund Status Values
- `none`: No refund processed
- `partial`: Partial refund (amount < original amount)
- `full`: Full refund (amount = original amount)

### Income Type Values
- `normal`: Regular income transaction
- `refund`: Income generated from a refund
- `reimbursement`: Income generated from a reimbursement

## Usage Examples

### Creating a Transaction with Precise Time
```go
transactionTime := time.Date(2024, 1, 15, 14, 30, 0, 0, time.UTC)
tx := Transaction{
    Amount:          100.00,
    Type:            TransactionTypeExpense,
    TransactionDate: transactionTime,
    TransactionTime: &transactionTime,
    LedgerID:        &ledgerID,
}
```

### Applying for Reimbursement
```go
reimbursementAmount := 80.00
expense.ReimbursementStatus = "pending"
expense.ReimbursementAmount = &reimbursementAmount
```

### Processing a Refund
```go
refundAmount := 50.00
incomeID := uint(200)
expense.RefundStatus = "partial"
expense.RefundAmount = &refundAmount
expense.RefundIncomeID = &incomeID
```

### Creating Linked Income Record
```go
refundIncome := Transaction{
    Type:                  TransactionTypeIncome,
    Amount:                50.00,
    IncomeType:            "refund",
    OriginalTransactionID: &originalExpenseID,
    LedgerID:              originalExpense.LedgerID, // Same ledger as original
}
```

## Requirements Validation

This implementation validates the following requirements:
- ✓ 3.10: Multi-ledger transaction association
- ✓ 5.2: Precise time recording
- ✓ 8.4-8.9: Reimbursement workflow
- ✓ 8.10-8.18: Refund workflow
- ✓ 8.19-8.22: Original transaction linking
- ✓ 8.28: Ledger consistency for refund/reimbursement income

## Next Steps

The Transaction model is now ready for:
1. Backend API implementation for reimbursement operations (Task 5.1)
2. Backend API implementation for refund operations (Task 5.2)
3. Frontend integration with transaction forms
4. Property-based testing for transaction workflows

## Notes

- All pointer fields (`*uint`, `*float64`, `*time.Time`) are optional and can be nil
- Default values for status fields are set by GORM on database insert
- Foreign key constraints are commented out in the migration to avoid circular reference issues
- The implementation maintains backward compatibility with existing transactions