/// <summary>
 /// Asynchronously loads first record from query.
 /// Returns <c>default(TSource)</c>, if query doesn't return any records.
 /// </summary>
 /// <typeparam name="TSource">Query element type.</typeparam>
 /// <param name="source">Source query.</param>
 /// <param name="token">Optional asynchronous operation cancellation token.</param>
 /// <returns>First record from query results or <c>default(TSource)</c> for empty resultset.</returns>
 public static Task <TSource> FirstOrDefaultAsyncLinqToDB <TSource>(
     this IQueryable <TSource> source,
     CancellationToken token = default)
 => AsyncExtensions.FirstOrDefaultAsync(source.ToLinqToDB(), token);
 /// <summary>
 /// Asynchronously loads first record from query, filtered using provided predicate.
 /// Returns <c>default(TSource)</c>, if query doesn't return any records.
 /// </summary>
 /// <typeparam name="TSource">Query element type.</typeparam>
 /// <param name="source">Source query.</param>
 /// <param name="predicate">Query filter predicate.</param>
 /// <param name="token">Optional asynchronous operation cancellation token.</param>
 /// <returns>First record from query results or <c>default(TSource)</c> for empty resultset.</returns>
 public static Task <TSource> FirstOrDefaultAsyncLinqToDB <TSource>(
     this IQueryable <TSource> source,
     Expression <Func <TSource, bool> > predicate,
     CancellationToken token = default)
 => AsyncExtensions.FirstOrDefaultAsync(source.ToLinqToDB(), predicate, token);
 /// <summary>
 /// Returns the first element of a sequence, or a default value if the sequence contains no elements
 /// </summary>
 /// <typeparam name="TSource">The type of the elements of source</typeparam>
 /// <param name="source">Source</param>
 /// <param name="predicate">Predicate</param>
 /// <returns>default(TSource) if source is empty; otherwise, the first element in source</returns>
 public static Task <TSource> FirstOrDefaultAsync <TSource>(this IQueryable <TSource> source,
                                                            Expression <Func <TSource, bool> > predicate = null)
 {
     return(predicate == null?AsyncExtensions.FirstOrDefaultAsync(source) : AsyncExtensions.FirstOrDefaultAsync(source, predicate));
 }