Rob’s SprintResult Transformers

time to read 5 min | 850 words

By far the most confusing feature in RavenDB has been the index’s Transform Result. We introduced this feature to give the user the ability to do server side projections, including getting data from other documents.

Unfortunately, when we introduced this feature, we naturally added it to the index, and that cause a whole lot of confusion. In particular, people seemed to have a very hard time distinguishing between what get indexed and is searchable and the output of the index. To make matters worse, we also had major issues with how to determine the input of the TransformResults function. In short, the entire thing works, but from the point of view of an external user, that is really something that is very finicky and hard to get.

Instead, during Rob’s sprint, we have introduced a totally new concept. Stand-alone Result Transformers.

Here is what they look like:

public class OrdersStatsTransfromer : AbstractTransformerCreationTask<Order>
    public OrdersStatsTransfromer()
        TransformResults = orders =>
                           from order in orders
                           select new
                               CustomerName = LoadDocument<Customer>(order.CustomerId).Name,
                               LinesCount = order.Lines.Count

And yes, they are quite intentionally modeled to be very similar to the way you would define them up to now, but outside of the index.

Now, why is that important? Because now you can apply a Transform Results on the server side without being tied to a customer.

For example, let us see how we can make use of this new feature:

var customerOrders = session.Query<Order>()
    .Where(x => x.CustomerId == "customers/123")
    .TransformWith<OrdersStatsTransfromer, OrderViewModel>()

This separation between the result transformer and the index means that we can apply it to things like automatic indexes as well.

In fact, we can apply it during load:

var ovm = session.Load<OrdersStatsTransfromer, OrderViewModel>("orders/1");

There are a whole bunch of other goodies in there, as well. We made sure that now you don’t have to worry about the inputs to the transform. We will automatically use the right values when you access them, based on whatever you stored the field in the index or if it is accessible on the document.

All in all, this is a very major step forward, and it makes it drastically easier to use Result Transformers in various ways.

More posts in "Rob’s Sprint" series:

  1. (08 Mar 2013) The cost of getting data from LevelDB
  2. (07 Mar 2013) Result Transformers
  3. (06 Mar 2013) Query optimizer jumped a grade
  4. (05 Mar 2013) Faster index creation
  5. (04 Mar 2013) Indexes and the death of temporary indexes
  6. (28 Feb 2013) Idly indexing