move this:
```
// calculate path
NSArray* paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString* documentsDirectory = [paths objectAtIndex:0];

// create the Documents directory if it doesn't exist
BOOL isDir;
if (![[NSFileManager defaultManager] fileExistsAtPath:documentsDirectory isDirectory:&isDir]) {
    NSError *error = nil;
    [[NSFileManager defaultManager] createDirectoryAtPath:documentsDirectory
      withIntermediateDirectories:YES attributes:nil error:&error];
}

// the filename is based on this store name
_file = [documentsDirectory stringByAppendingPathComponent:config.name];
```

into a private method, that just returns the NSString file;