lesson6

Unit 2, Lesson 6 - Goodbye Transformers, Welcome Server-Side Projections

In the previous lesson, you learned how to create shorter results from the server using the LoadDocument server-side projections.

In this lesson, I will show you how to get shape results using server-side projections.

What Happened with Transformers?

If you know RavenDB 3.5, you are probably looking for how to implement Transformers. Bad news: They are gone! Good news: You will not miss them.

Transformers were removed and substituted by a server-side projection support. Methods like TransformWith are no longer available and simple Select should be used instead.

Easy Start with Server-Side Projections

Instead of pulling full documents in query results, you can just grab some pieces of data from documents. You can also transform the projected results.

Let me share a short example:

// request Name, City and Country 
// for all entities from 'Companies' collection
var results = session
    .Query<Company>()
    .Select(x => new
    {
        Name = x.Name,
        City = x.Address.City,
        Country = x.Address.Country
    })
    .ToList();

The related RQL is pretty simple as well:

from Companies
select Name, Address.City as City, Address.Country as Country

Another example? Here we go:

var results = (from e in session.Query<Employee>()
               select new
               {
                   FullName = e.FirstName + " " + e.LastName,
               }).ToList();

And here it is the RQL:

from Employees as e
select {
    FullName : e.FirstName + " " + e.LastName
}

Getting Some More Advanced Results Using Functions

Let's do something more complex.

var results = (from e in session.Query<Employee>()
               let format = 
                   (Func<Employee, string>)(p => 
                        p.FirstName + " " + p.LastName)
               select new
               {
                   FullName = format(e)
               }).ToList();

We just created a function that will run on the server-side.

Let's look at the RQL.

declare function output(e) {
	var format = function(p){ return p.FirstName + " " + p.LastName; };
	return { FullName : format(e) };
}
from Employees as e select output(e)

Yes, we can define functions when coding with RQL! These functions are pure JavaScript.

Great job!

To learn more about this functionality, I recommend you to read to read our article that covers the projection functionality which can be found here.

Let's move onto Lesson 7.