# Custom Task Status DELETE Endpoint Implementation

## Overview
Implemented the DELETE endpoint for removing custom task statuses from projects with comprehensive validation and task reassignment support.

## Endpoint Details

### DELETE `/projects/{project_id}/task-statuses/{status_id}`

**Authentication Required:** Coordinator or Admin

**Query Parameters:**
- `reassign_to_status_id` (optional): Status ID to reassign tasks to if the status being deleted is in use

**Response:** 200 OK with CustomTaskStatusResponse containing:
- Success message
- All remaining statuses (system + custom)
- Updated default status ID

## Implementation Features

### 1. Status In-Use Check
- Queries all tasks in the project to check if any use the status being deleted
- Returns detailed error (422) if status is in use and no reassignment provided:
  ```json
  {
    "error": "Cannot delete status 'X' because it is currently in use by N task(s)",
    "status_id": "custom_abc123",
    "status_name": "In Review",
    "task_count": 5,
    "task_ids": [1, 2, 3, 4, 5]
  }
  ```

### 2. Task Reassignment
- Supports optional `reassign_to_status_id` query parameter
- Validates reassignment target exists (can be system or custom status)
- Prevents reassigning to the same status being deleted
- Automatically updates all affected tasks to the new status
- Includes reassignment count in success message

### 3. Default Status Management
- Detects if the status being deleted is the default status
- Automatically assigns the first remaining custom status as the new default
- Ensures there's always a default status after deletion

### 4. Last Status Protection
- Prevents deletion of the last custom status
- Returns 422 error with clear message
- Ensures at least one custom status always remains

### 5. Error Handling
- 404: Status not found
- 404: Project not found
- 400: Invalid reassignment status ID
- 422: Status in use without reassignment
- 422: Attempting to delete last status
- 500: Database operation failure

## Code Structure

```python
@router.delete("/{project_id}/task-statuses/{status_id}")
async def delete_custom_task_status(
    project_id: int,
    status_id: str,
    reassign_to_status_id: Optional[str] = Query(None),
    db: Session = Depends(get_db),
    current_user: User = Depends(require_coordinator_or_admin)
):
    # 1. Verify project exists
    # 2. Load custom statuses from JSON
    # 3. Find status to delete
    # 4. Prevent deletion of last status
    # 5. Check if status is in use
    # 6. Handle reassignment if needed
    # 7. Auto-assign new default if needed
    # 8. Update database with flag_modified
    # 9. Return success response with all statuses
```

## Test Coverage

Comprehensive test suite in `test_delete_custom_task_status.py`:

1. ✅ Delete unused custom status
2. ✅ Delete status in use without reassignment (error)
3. ✅ Delete status in use with reassignment
4. ✅ Delete default status (auto-assign new default)
5. ✅ Prevent deletion of last status
6. ✅ Delete non-existent status (error)

## Requirements Validation

✅ **3.1**: Check if status is in use by any tasks
✅ **3.2**: Return error with task count and IDs if in use
✅ **3.3**: Support optional reassignment of tasks to another status
✅ **3.4**: Auto-assign new default if deleting default status
✅ **3.5**: Prevent deletion of last status

## Database Considerations

- Uses `flag_modified()` for JSON column updates (required for SQLAlchemy to detect changes)
- Transactional: All changes (status deletion + task reassignments) happen in one transaction
- Rollback on any error to maintain data consistency

## Usage Examples

### Delete unused status
```bash
DELETE /projects/1/task-statuses/custom_abc123
```

### Delete status with reassignment
```bash
DELETE /projects/1/task-statuses/custom_abc123?reassign_to_status_id=not_started
```

### Delete status with reassignment to another custom status
```bash
DELETE /projects/1/task-statuses/custom_abc123?reassign_to_status_id=custom_xyz789
```

## Integration Notes

- Works seamlessly with existing custom status endpoints (GET, POST, PUT)
- Maintains consistency with system statuses (cannot delete system statuses)
- Properly updates the AllTaskStatusesResponse to reflect changes
- Frontend can use the returned `all_statuses` to update UI immediately

## Future Enhancements

Potential improvements for future iterations:
- Bulk delete with single reassignment target
- Soft delete with archive functionality
- Status usage analytics before deletion
- Undo/restore deleted statuses