# Upsert Fix for Reference Table Imports

## Issue
When attempting to re-import CSV files for reference tables (like `trnstype`, `trnslkup`, `footers`, etc.), the application encountered UNIQUE constraint errors because the import functions tried to insert duplicate records:

```
Fatal error: (sqlite3.IntegrityError) UNIQUE constraint failed: trnstype.t_type
```

## Root Cause
The original import functions used `bulk_save_objects()` which only performs INSERT operations. When the same CSV was imported multiple times (e.g., during development, testing, or data refresh), the function attempted to insert records with primary keys that already existed in the database.

## Solution
Implemented **upsert logic** (INSERT or UPDATE) for all reference table import functions:

### Modified Functions
1. `import_trnstype()` - Transaction types
2. `import_trnslkup()` - Transaction lookup codes
3. `import_footers()` - Footer templates
4. `import_filestat()` - File status definitions
5. `import_employee()` - Employee records
6. `import_gruplkup()` - Group lookup codes
7. `import_filetype()` - File type definitions
8. `import_fvarlkup()` - File variable lookups
9. `import_rvarlkup()` - Rolodex variable lookups

### Key Changes

#### Before (Insert-Only)
```python
record = TrnsType(
    t_type=t_type,
    t_type_l=clean_string(row.get('T_Type_L')),
    header=clean_string(row.get('Header')),
    footer=clean_string(row.get('Footer'))
)
batch.append(record)
if len(batch) >= BATCH_SIZE:
    db.bulk_save_objects(batch)
    db.commit()
```

#### After (Upsert Logic)
```python
# Check if record already exists
existing = db.query(TrnsType).filter(TrnsType.t_type == t_type).first()

if existing:
    # Update existing record
    existing.t_type_l = clean_string(row.get('T_Type_L'))
    existing.header = clean_string(row.get('Header'))
    existing.footer = clean_string(row.get('Footer'))
    result['updated'] += 1
else:
    # Insert new record
    record = TrnsType(
        t_type=t_type,
        t_type_l=clean_string(row.get('T_Type_L')),
        header=clean_string(row.get('Header')),
        footer=clean_string(row.get('Footer'))
    )
    db.add(record)
    result['inserted'] += 1

result['success'] += 1

# Commit in batches for performance
if result['success'] % BATCH_SIZE == 0:
    db.commit()
```

## Benefits

1. **Idempotent Imports**: Can safely re-run imports without errors
2. **Data Updates**: Automatically updates existing records with new data from CSV
3. **Better Tracking**: Result dictionaries now include:
   - `inserted`: Count of new records added
   - `updated`: Count of existing records updated
   - `success`: Total successful operations
4. **Error Handling**: Individual row errors don't block the entire import

## Testing

To verify the fix works:

1. Import a CSV file (e.g., `trnstype.csv`)
2. Import the same file again
3. The second import should succeed with `updated` count matching the first import's `inserted` count

## Performance Considerations

- Still uses batch commits (every BATCH_SIZE operations)
- Individual record checks are necessary to prevent constraint violations
- For large datasets, this is slightly slower than bulk insert but provides reliability

## Future Enhancements

Consider implementing database-specific upsert operations for better performance:
- SQLite: `INSERT OR REPLACE`
- PostgreSQL: `INSERT ... ON CONFLICT DO UPDATE`
- MySQL: `INSERT ... ON DUPLICATE KEY UPDATE`

