Ravendb: map-reduce on two relating documents - mapreduce

Question

In my ravendb I have 2 documents: Country and City
City document looks like this
Id
Name
CountryId
Country document looks like this
Id
Name
At the moment I have an index where I retrieve all cities as a list and this works.
But I would rather want to retrieve all cities grouped by countries
This is what I have
public class City_ByCountry
{
public string CityId { get; set; }
public string CityName { get; set; }
public string CountryName { get; set; }
}
Map = (city => from cit in city
let cou = LoadDocument<Country>(cit.CountryId)
select new City_ByCountry
{
CityId = cit.Id,
CityName = cit.Name,
CountryName = cou.Name
});
This works but gives me a list of all cities (id, name, countryName)
But I want a list like this
CountryName [
List with cities]
CountryName [
List with cities]
etc
Can I do this with a reduce on the result? Or what is the correct way to do this?

Answer 1

I think with a reduce this is possible. See Ayende's post Awesome indexing with RavenDB on how to perform advanced indexing.
I tried to modify Ayende's example to match your needs (just on Notepad, so I don't know if it even compiles):
public class City_ByCountry : AbstractIndexCreationTask<City, City_ByCountry.ReduceResult>
{
public class ReduceResult
{
public string CountryId { get; set; }
public string CountryName { get; set; }
public City[] Cities { get; set; }
}
public City_ByCountry()
{
Map = cities =>
from city in cities
select new
{
CountryId= city.CountryId,
CountryName = LoadDocument<Country>(city.CountryId),
Cities = new [] { city }
};
Reduce = cities =>
from city in cities
group city by city.CountryId
into g
select new
{
CountryId = g.Key,
CountryName = g.First().CountryName,
Cities = g.SelectMany(x => x.Cities)
};
}
}