Linq query help

Question

I'm attempting to write a linq query which uses several tables of related data and have gotten stuck.

The expected result: I need to return the three most populous metropolitan areas per region by population descending.

tables w/sample data:

MetroAreas -- ID, Name
2, Greater New York

Cities -- ID, Name, StateID
1293912, New York City, 10

CityPopulations -- ID, CityID, CensusYear, Population
20, 1293912, 2008, 123456789
21, 1293912, 2007, 123454321

MetroAreaCities -- ID, CityID, MetroAreaID
1, 1293912, 2

States -- ID, Name, RegionID
10, New York, 5

Regions -- ID, Name
5, Northeast

I start with the metro areas. Join the MetroAreaCities to get city IDs. Join Cities to get state IDs. Join States to get the region ID. Join regions so I can filter with a where. I get stuck when I try to include CityPopulations. I only want the three most populous metro areas for a given region. Doing a simple join on the cityPopulations returns a record per year.

(Here's what I have so far, this query was written for SubSonic 3):

return from p in GeoMetroArea.All()  
       join q in GeoMetroAreaCity.All() on p.ID equals q.MetroAreaID  
       join r in GeoCity.All() on q.CityID equals r.ID  
       join s in GeoState.All() on r.StateID equals s.ID  
       join t in GeoRegion.All() on s.RegionID equals t.ID  
       where t.ID == regionObjectPassedToMethod.ID  
       select p;  

Can anyone help me with this query or point me in the right direction? Thank you very very much.

Answer 1

I haven't compiled it, but this should get you close:

var regionID = 5;

var year = (from c in GeoCityPopulation.All()
            select c.CensusYear
           ).Max();

var metros =
    // States in Region
    from s in GeoStateAll()
    where s.RegionID == regionID
    // Cities in State
    join c in GeoCity.All() on s.CityID equals c.ID
    // Metro Area for City
    join mc in GeoMetroAreaCity.All() on c.ID equals mc.CityID
    // Population for City
    join cp in GeoCityPopulation.All() on c.ID equals cp.CityID
    where cp.CensusYear = year
    // Group the population values by Metro Area
    group cp.Population by mc.MetroAreaID into g
    select new
    {
        MetroID = g.Key,      // Key = mc.MetroAreaID
        Population = g.Sum()  // g = seq. of Population values
    } into mg
    // Metro for MetroID
    join m in GeoMetroArea.All() on mg.MetroID equals m.ID
    select new { m.Name, mg.Population };