Blog_RavenDBTransformer.cs

public class DocumentTransformer : AbstractTransformerCreationTask<Document>
{
    public class Result
    {
        public string Id { get; set; }
        public string[] ExtractedIds { get; set; }
    }

    public DocumentTransformer()
    {
        TransformResults = docs => from doc in docs
            select new Result
            {
                Id = doc.Id,
                ExtractedIds = doc.SomeCollection.ToArray(),
            };
    }
}