Polymorphic entities in Room

I think you have a number of options here:

Option 1

You use a single table to store all MediaItems and you use a discriminator column to make the difference between a video and a photo. You have a single DAO method that performs the query, applies the order by and returns a Cursor. Then you can use your existing cursor manipulation logic to return a List<MediaItem> It can look like this:

@Dao
public abstract class MediaItemDao() {

    @Query("you query here")
    protected Cursor getByAlbumIdInternal(int albumId);

    public List<MediaItem> getByAbumId(int albumId) {
        Cursor cursor = null;
        try{
            List<MediaItem> mediaList = new ArrayList<>();
            cursor = getByAlbumIdInternal(albumId);
            // looping through all rows and adding to list
            if (cursor.moveToFirst()){
                do {
                    // use the discriminator value here
                    int mediaType = cursor.getInt(cursor.getColumnIndex("you discriminator column name here"));
                    MediaItem mediaItem = null;
                    if (mediaType == 0) {
                        mediaItem = new PhotoMediaItem(cursor.getInt(0), null, palbumId);
                    } else if (mediaType == 1) {
                        mediaItem = new VideoMediaItem(cursor.getInt(0), null, palbumId, cursor.getLong(3));
                    }
                    mediaList.add(mediaItem);
                } while (cursor.moveToNext());
            }
            return mediaList;
        }
        finally  {
            if(cursor != null){
                cursor.close();
            }
        }
    }
}

Option 2

You use two different tables to store VideosMediaItem and PhotosMediaItem. You have a MediaItemDao that has two internal methods to perform the queries and a single public method that merges the two result sets together and applies the sorting in java code. It can look like this:

@Dao
public abstract class MediaItemDao() {

    @Query("your query to get the videos, no order by")
    protected List<VideoMediaItem> getVideosByAlbumId(int albumId);

    @Query("your query to get the photos, no order by")
    protected List<PhotosMediaItem> getPhotosByAlbumId(int albumId);

    @Transaction
    public List<MediaItem> getByAlbumId(int albumId) {
        final List<MediaItem> mediaItems = new LinkedList<>();
        mediaItems.add(getVideosByAlbumId(albumId));
        mediaItems.add(getPhotosByAlbumId(albumId));
        Collections.sort(mediaItems, <you can add a comparator here>);
        return mediaItems;
    }
}

EDIT: How to leverage live data for this option?

As I mentioned, you should use LiveData as the return type of your protected methods so you get notified for underlying changes on those tables. So they should look like this:

protected LiveData<List<VideoMediaItem>> getVideosByAlbumId(int albumId);

protected LiveData<List<PhotosMediaItem>> getPhotosByAlbumId(int albumId);

In order to return a single LiveData to the client, you should zip the outputs of those two methods into a single stream. You can achieve this using a custom MediatorLiveData implementation. It may look like this:

public class ZipLiveData<T1, T2, R> extends MediatorLiveData<R> {

    private T1 mLastLeft;
    private T2 mLastRight;
    private Zipper<T1, T2, R> mZipper;

    public static final <T1, T2, R> LiveData<R> create(@NonNull LiveData<T1> left, @NonNull LiveData<T2> right, Zipper<T1, T2, R> zipper) {
        final ZipLiveData<T1, T2, R> liveData = new ZipLiveData(zipper);
        liveData.addSource(left, value -> {
            liveData.mLastLeft = value;
            update();
        });
        liveData.addSource(right, value -> {
            liveData.mLastRight = value;
            update();
        });
        return liveData;
    }

    private ZipLiveData(@NonNull Zipper<T1, T2, R> zipper) {
        mZipper = zipper;
    }

    private update() {
        final R result = zipper.zip(mLastLeft, mLastRight);
        setValue(result);
    }

    public interface Zipper<T1, T2, R> {

        R zip(T1 left, T2 right);

    }
}

Then you just use it in your repository public method like this:

public List<MediaItem> getByAlbumId(int albumId) {
    return ZipLiveData.create(
        getVideosByAlbumId(albumId),
        getPhotosByAlbumId(albumId),
        (videos, photos) -> {
            final List<MediaItem> mediaItems = new LinkedList<>();
            mediaItems.add(videos);
            mediaItems.add(photos);
            Collections.sort(mediaItems, <you can add a comparator here>);
            return mediaItems;
        }
}

Option 3

This is applicable only if you have a repository layer in place.

You use two different tables to store VideosMediaItem and PhotosMediaItem. You have a DAO class for each one. You have a repository that depends on the both DAOs and combines the result sets, applying the sorting. It can look like this:

@Dao
public abstract class VideosMediaItemDao {

    @Query("your query to get the videos, no order by")
    public List<VideoMediaItem> getByAlbumId(int albumId);

}

@Dao
public abstract class PhotosMediaItemDao {

    @Query("your query to get the photos, no order by")
    public List<PhotosMediaItem> getByAlbymId(int albumId);

}

public interface MediaItemRepository {

    public List<MediaItem> getByAlbumId(int albumId);

}

class MediaItemRepositoryImpl {

    private final VideosMediaItemDao mVideoDao;
    private final PhotosMediaItemDao mPhotoDao;

    MediaItemRepositoryImpl(VideosMediaItemDao videoDao, PhotosMediaItemDao photoDao) {
        mVideoDao = videoDao;
        mPhotoDao = photoDao;
    }

    @Override
    public List<MediaItem> getByAlbumId(int albumId) {
        final List<MediaItem> mediaItems = new LinkedList<>();
        mediaItems.add(mVideoDao.getByAlbumId(albumId));
        mediaItems.add(mPhotoDao.getByAlbumId(albumId));
        Collections.sort(mediaItems, <you can add a comparator here>);
        return mediaItems;
    }

}