For files you can use the file command: :file:\`db/itemmapper.php`

The line is not really accurate. The itemmapper file should be in the db directory but you can change it by adjusting the namespace.
So if you put the itemmapper.php in the folder databases/ for instance, the namespace of the itemmapper must be: 

```php
    namespace OCA\YourApp\Databases;
    class ItemMapper { ...
```

And you would require it by using:

```php
    use \OCA\YourApp\Datases\ItemMapper as ItemMapper;
```