Results Transformers

Results Transformers have been introduced in RavenDB 2.5 to give the user the ability to do a server side projections (with possibility to load data from other documents).

Main features of the Result Transformers are:

I. Stand-alone, separated from Index.

public class Order
{
	public DateTime OrderedAt { get; set; }

	public Status Status { get; set; }

	public string CustomerId { get; set; }

	public IList<OrderLine> Lines { get; set; }
}

public class OrderStatisticsTransformer : AbstractTransformerCreationTask<Order>
{
	public OrderStatisticsTransformer()
	{
		TransformResults = orders => from order in orders
									 select new
											{
												order.OrderedAt,
												order.Status,
												order.CustomerId,
												CustomerName = LoadDocument<Customer>(order.CustomerId).Name,
												LinesCount = order.Lines.Count
											};
	}
}

II. User can use them on index results on demand.

What does it mean? It means if you want load whole Order you can do so by not using a transformer in the query:

IList<Order> orders = session.Query<Order>()
	.Where(x => x.CustomerId == "customers/1")
	.ToList();

or if you want to get your transformed results then you can execute query as follows:

public class OrderStatistics
{
	public DateTime OrderedAt { get; set; }

	public Status Status { get; set; }

	public string CustomerId { get; set; }

	public string CustomerName { get; set; }

	public int LinesCount { get; set; }
}

IList<OrderStatistics> statistics = session.Query<Order>()
	.TransformWith<OrderStatisticsTransformer, OrderStatistics>()
	.Where(x => x.CustomerId == "customers/1")
	.ToList();

III. Can be used with automatic indexes.

OrderStatistics statistic = session.Load<OrderStatisticsTransformer, OrderStatistics>("orders/1");