What I meant here is you need to build your test upon your PR (which solve the problem of always returning the default gridfs collection). In #766 you changed:
```php
$grid = $this->connection->getGridFS();
```
to:
```php
$grid = $this->connection->getGridFS($this->_config['gridPrefix']);
```
So your mock should be representative of the class you are mocking. I don't think you'll be able to build valid tests otherwise. This way you only need to test if passed parameter is correct over different CRUD operations.