public class Batch
{
    public Guid Id { get; set; }
    public Item[] Items { get; set; }
}

public class Item
{
    public int Id { get; set; }
    public string Description { get; set; }
}

Map = batches =>
    from batch in batches
    from item in batch.Items
    select new Result
        {
             Description = item.Description,
             ItemIds = new[] {item.Id}
        };

Reduce = results =>
    from result in results
    group result by result.Description into g
    select new Result
        {
             Description = g.Key,
             ItemIds = g.SelectMany(x => x.ItemIds).ToArray()
        };

Map = batches =>
     from batch in batches
     from item in batch.Items
     select new Result
         {
             Description = item.Description,
             ItemIdsAsString = item.Id.ToString(),
         };

Reduce = results =>
     from result in results
     group result by result.Description into g
     select new Result
         {
             Description = g.Key,
             ItemIdsAsString = string.Join("|", g.Select(x => x.ItemIdsAsString)),
         };