/// <summary>
/// Asynchronously loads first record from query.
/// Throws exception, if query doesn't return exactly one record.
/// </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.</returns>
public static Task <TSource> SingleAsyncLinqToDB <TSource>(
this IQueryable <TSource> source,
CancellationToken token = default)
=> AsyncExtensions.SingleAsync(source.ToLinqToDB(), token);
/// <summary>
/// Asynchronously loads first record from query, filtered using provided predicate.
/// Throws exception, if query doesn't return exactly one record.
/// </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.</returns>
public static Task <TSource> SingleAsyncLinqToDB <TSource>(
this IQueryable <TSource> source,
Expression <Func <TSource, bool> > predicate,
CancellationToken token = default)
=> AsyncExtensions.SingleAsync(source.ToLinqToDB(), predicate, token);
/// <summary>
/// Returns the only element of a sequence that satisfies a specified condition,
/// and throws an exception if more than one such element exists
/// </summary>
/// <typeparam name="TSource"></typeparam>
/// <param name="source">An sequence to return a single element from</param>
/// <param name="predicate">A function to test an element for a condition</param>
/// <returns>The single element of the input sequence that satisfies the condition in predicate</returns>
public static Task <TSource> SingleAsync <TSource>(this IQueryable <TSource> source,
Expression <Func <TSource, bool> > predicate = null)
{
return(predicate == null?AsyncExtensions.SingleAsync(source) : AsyncExtensions.SingleAsync(source, predicate));
}