Junction

@Entity(primaryKeys = {"pId", "sId"})
public class PlaylistSongXRef {
    val pId: Int,
    val sId: Int
}
public class PlaylistWithSongs {
    @Embedded
    val playlist: Playlist
    @Relation(
            parentColumn = "playlistId",
            entity = Song::class,
            entityColumn = "songId",
            associateBy = Junction(
                    value = PlaylistSongXRef::class,
                    parentColumn = "pId",
                    entityColumn = "sId")
    )
    val songs: List<String>
}

@Dao
public interface MusicDao {
    @Query("SELECT * FROM Playlist")
    val getAllPlaylistsWithSongs(): List<PlaylistWithSongs>
}