internal static class Extensions
{
    public static IEnumerable<string> GetPrimaryKeyForType(this Type elementType)
    {
        return from PropertyDescriptor property in TypeDescriptor.GetProperties(elementType)
               where null != property.Attributes[typeof(KeyAttribute)]
               select property.Name;
    }
    
    private static IOrderedQueryable SortCore(IQueryable source, string propertyName, ListSortDirection sortDirection, string methodName)
    {
        if (ListSortDirection.Descending == sortDirection)
        {
            methodName += "Descending";
        }

        var p = Expression.Parameter(source.ElementType, "p");
        var prop = Expression.Property(p, propertyName);
        var body = Expression.Lambda(prop, p);

        var call = Expression.Call(typeof(Queryable), methodName,
            new[] { p.Type, prop.Type },
            source.Expression, Expression.Quote(body));

        return (IOrderedQueryable)source.Provider.CreateQuery(call);
    }
    
    public static IOrderedQueryable Sort(this IQueryable source, IEnumerable<Tuple<string, ListSortDirection>> sortBy)
    {
        if (null == source) throw new ArgumentNullException("source");
        if (null == sortBy) throw new ArgumentNullException("sortBy");

        IOrderedQueryable result;
        using (var enumerator = sortBy.Where(t => null != t).GetEnumerator())
        {
            if (!enumerator.MoveNext())
            {
                throw new ArgumentException(Resources.Error_EmptyEnumerable, "sortBy");
            }

            var tuple = enumerator.Current;
            result = SortCore(source, tuple.Item1, tuple.Item2, "OrderBy");
            while (enumerator.MoveNext())
            {
                tuple = enumerator.Current;
                result = SortCore(result, tuple.Item1, tuple.Item2, "ThenBy");
            }
        }

        return result;
    }
    
    public static long LongCount(this IQueryable source)
    {
        if (null == source) throw new ArgumentNullException("source");

        Expression call = Expression.Call(typeof(Queryable), "LongCount",
            new[] { source.ElementType },
            source.Expression);

        return source.Provider.Execute<long>(call);
    }
    
    public static IQueryable Take(this IQueryable source, int count)
    {
        if (null == source) throw new ArgumentNullException("source");

        var call = Expression.Call(typeof(Queryable), "Take",
            new[] { source.ElementType },
            source.Expression, Expression.Constant(count));

        return source.Provider.CreateQuery(call);
    }
    
    public static IQueryable SkipLong(this IQueryable source, long count)
    {
        if (null == source) throw new ArgumentNullException("source");

        while (0L < count)
        {
            int toSkip;
            if (int.MaxValue < count)
            {
                count -= int.MaxValue;
                toSkip = int.MaxValue;
            }
            else
            {
                toSkip = (int)count;
                count = 0L;
            }

            var call = Expression.Call(typeof(Queryable), "Skip",
                new[] { source.ElementType },
                source.Expression, Expression.Constant(toSkip));

            source = source.Provider.CreateQuery(call);
        }

        return source;
    }
    
    public static IEnumerable Materialize(this IQueryable source)
    {
        if (null == source) throw new ArgumentNullException("source");
        return Enumerable.Cast<object>(source).ToList();
    }
}