Summing in multi-level relationship - linq

Question

Using EF code first, I have the following 4 entities
public class Item {
public int Id { get; set; }
public string Name { get; set; }
}
public class Location {
public int Id { get; set; }
public string Name { get; set; }
}
public class InventoryAdjustment {
public int Id { get; set; }
public virtual Location Location { get; set; }
public virtual ICollection<AdjustmentLine> Lines { get; set; }
}
public class AdjustmentLine {
public int Id { get; set; }
public virtual Item Item { get; set; }
public int Quantity { get; set; }
}
What I am trying to do is to get the sum of all inventory adjustments for each item at each location using minimal database round-trips.
The best I achieved so far is:
using (var db = new InventoryContext()) {
var items = db.Items.ToList();
var locations = db.Locations.ToList();
foreach (var item in items) {
Console.WriteLine(item.Name+":");
foreach (var location in locations) {
Console.Write("\t" + location.Name + ": ");
var qty = db.InventoryAdjustments
.Where(p => p.Location.Id == location.Id)
.SelectMany(p => p.Lines)
.Where(p => p.Item.Id == item.Id)
.Select(p => (int?)p.Quantity)
.Sum();
Console.WriteLine(qty ?? 0);
}
}
Console.Read();
}
The above outputs:
Item1:
Location1: 2
Location2: 12
Location3: 21
Item2:
Location1: 4
Location2: 0
Location3: 0
Item3:
Location1: 1
Location2: 17
Location3: 0
But with 3 items and 3 locations in the database, the above code causes 11 calls to the database. 2 for getting items and locations, and 9 for calculating the sum of quantity.
Is there a better way to get the sum with the least amount of round-trips?

Answer 1

This should probably work:
using (var db = new InventoryContext())
{
var items = db.Items.ToList();
var locations = db.Locations.ToList();
items
.Select(item =>
{
Console.WriteLine(item.Name + ":");
return item;
})
.SelectMany(item => locations.Select(location => new { item, location }))
.GroupJoin(
db.InventoryAdjustments
.SelectMany(
inventoryAdjustment => inventoryAdjustment.Lines.Select(
adjustmentLine => new { key = new { locationId = inventoryAdjustment.Location.Id, itemId = adjustmentLine.Item.Id }, adjustmentLine.Quantity }
)
),
x => new { locationId = x.location.Id, itemId = x.item.Id },
y => y.key,
(x, y) =>
{
Console.WriteLine("\t {0}: {1}", x.location.Name, y.Sum(a => a.Quantity));
return 0;
}
).ToList();
Console.Write("\nPress any key...");
Console.ReadKey();
}