/// <summary>
/// Returns average track rating for the specified track.
/// </summary>
/// <param name="trackId">
/// The track id.
/// </param>
/// <returns>
/// The average track rating for the specified track.
/// </returns>
public double GetAverageMark(int trackId)
{
return(CurrentDbSet.Any(v => v.TrackId == trackId) ? CurrentDbSet.Where(v => v.TrackId == trackId).Average(v => v.Mark) : 0d);
}