Angularjs: Best way to deal with foreign keys in items data

I have files and folders. Folders contain files.

The JSON that I get from the server looks like this:

folder: {id: 5, name: "my stuff" }
file: {id: 3, name: "my file", folderId: 5}
file: {id: 5, name: "my file 2", folderId: 5}

Whats the best way to build a Folder - File structure? Nest the files in the folder like:

{id: 5, name: "my stuff" files: [
    {id: 3, name: "my file", folderId: 5},
    {id: 5, name: "my file 2", folderId: 5}
]},
{id: 6, name: "other things" files: [
    {id: 1, name: "my file", folderId: 6},
    {id: 2, name: "my file 2", folderId: 6}
]}

Or use a filter that only returns the files that belong to a folder id? Like:

<li ng-repeat="folder in folders">
    <ul>
        <li ng-repeat="file in files | fileInFolder:folder.id"></li>
    </ul>
</li>

Files can be moved to different folders!