Session: Querying: How to perform projection?

The most common projection is done by using LINQ Select method.

Example I

// request 'FirstName' and 'LastName' from server
// and project it to anonymous class
var results = session
	.Query<Employee>()
	.Select(x => new
	{
		FirstName = x.FirstName,
		LastName = x.LastName
	})
	.ToList();

Example II

// request 'FirstName' and 'LastName' from server
// and project it to 'EmployeeFirstAndLastName'
List<EmployeeFirstAndLastName> results = session
	.Query<Employee>()
	.Select(x => new EmployeeFirstAndLastName
	{
		FirstName = x.FirstName,
		LastName = x.LastName
	})
	.ToList();

Detailed article about using transformers with queries can be found here.

This extension method uses reflection to extract all public fields and properties to fetch and perform projection to the requested type.

Example

// request all public fields/properties available 
// in 'EmployeeFirstAndLastName' ('FirstName' and 'LastName')
// and project it to instance of this class
List<EmployeeFirstAndLastName> results = session
	.Query<Employee>()
	.ProjectFromIndexFieldsInto<EmployeeFirstAndLastName>()
	.ToList();

OfType or As is a client-side projection. The easiest explanation of how it works is: take results that server returned and map them to given type. This may become useful when querying index that contains fields that are not available in mapped type.

Example

public class Products_BySupplierName : AbstractIndexCreationTask<Product>
{
	public class Result
	{
		public string Name { get; set; }
	}

	public Products_BySupplierName()
	{
		Map =
			products =>
			from product in products
			let supplier = LoadDocument<Supplier>(product.Supplier)
			select new
				       {
					       Name = supplier.Name
				       };
	}
}

// query index 'Products_BySupplierName' 
// return documents from collection 'Products' that have a supplier 'Norske Meierier'
// project them to 'Products'
List<Product> results = session
	.Query<Products_BySupplierName.Result, Products_BySupplierName>()
	.Where(x => x.Name == "Norske Meierier")
	.OfType<Product>()
	.ToList();