LINQ: Traverse up a hierarchy to retrieve parent hierarchy - linq

Question

I have some data which represents parent-child relationship on the same entity. Given a node, I need to find its entire upper hierarchy (parent, grand-parent, great grand-parent, etc..)
My entity is like this:
public partial class Location{
public int LocationId { get; set; }
public int? FkParentLocationId { get; set; }
..... more properties here.......
public virtual Location FkParentLocation { get; set; }
public virtual ICollection<Location> InverseFkParentLocation { get; set; }
}
I'm referring to the Hierarchy traverse implementation suggested here, but it works when you go down the hierarchy. How would retrieve the upper hierarchy using LINQ?
Sample data:
List<Location> locations = new List<Location> {
new Location { LocationId = 5, FkParentLocationId = 3, LocationName = "Windsor", LocationDisplayName = "Windsor"},
new Location { LocationId = 15, FkParentLocationId = 3, LocationName = "Hampshire", LocationDisplayName = "Hampshire" },
new Location { LocationId = 12, FkParentLocationId = 3, LocationName = "Sussex", LocationDisplayName = "Sussex"},
new Location { LocationId = 13, FkParentLocationId = 3, LocationName = "Willowood", LocationDisplayName = "Willowood"},
new Location { LocationId = 1, FkParentLocationId = 3, LocationName = "Gerbshire", LocationDisplayName = "Gerbshire"},
new Location { LocationId = 3, FkParentLocationId = 2, LocationName = "Lincoln", LocationDisplayName = "Lincoln"},
new Location { LocationId = 2, LocationName = "Mains", LocationDisplayName = "Mains" } };
Expected output: given location Id:5, I should get a list containing the locations 3 and 2 (as they are the parents).

Answer 1

Here is an approach you could use, demoed with a console app. Heavily borrowing from Jon Skeet.
using System;
using System.Collections.Generic;
using System.Linq;
namespace Locations
{
public partial class Location
{
public int LocationId { get; set; }
public int? FkParentLocationId { get; set; }
public virtual Location FkParentLocation { get; set; }
public virtual ICollection<Location> InverseFkParentLocation { get; set; }
public string LocationName { get; set; }
public string LocationDisplayName { get; set; }
}
class Program
{
static void Main(string[] args)
{
List<Location> locations = new List<Location> {
new Location { LocationId = 5, FkParentLocationId = 3, LocationName = "Windsor", LocationDisplayName = "Windsor"},
new Location { LocationId = 15, FkParentLocationId = 3, LocationName = "Hampshire", LocationDisplayName = "Hampshire" },
new Location { LocationId = 12, FkParentLocationId = 3, LocationName = "Sussex", LocationDisplayName = "Sussex"},
new Location { LocationId = 13, FkParentLocationId = 3, LocationName = "Willowood", LocationDisplayName = "Willowood"},
new Location { LocationId = 1, FkParentLocationId = 3, LocationName = "Gerbshire", LocationDisplayName = "Gerbshire"},
new Location { LocationId = 3, FkParentLocationId = 2, LocationName = "Lincoln", LocationDisplayName = "Lincoln"},
new Location { LocationId = 2, LocationName = "Mains", LocationDisplayName = "Mains" } };
var result = GetAncestorsIds(locations, 5);
foreach (var id in result)
{
System.Console.WriteLine(id);
}
}
private static IEnumerable<int> GetAncestorsIds(List<Location> locations, int id)
{
Location location = locations.SingleOrDefault(l => l.LocationId == id);
if(location != null)
{
while(location != null && location.FkParentLocationId != null)
{
location = locations.SingleOrDefault(l => l.LocationId == location.FkParentLocationId);
if(location != null)
{
yield return location.LocationId;
}
}
}
}
}
}
And this approach can be turned into your own Linq extension. Here's how it could look.
public static class MyExtensions
{
public static IEnumerable<int> GetAncestorIds<TSource>(this IEnumerable<TSource> source, Func<TSource, int> pk, Func<TSource, int?> fk, int id)
{
TSource currentObj = source.SingleOrDefault(s => pk(s) == id);
while(currentObj != null && fk(currentObj) != null)
{
currentObj = source.SingleOrDefault(s => pk(s) == fk(currentObj));
if(currentObj != null)
{
yield return pk(currentObj);
}
}
}
}
and then to call this for your scenario you would do this
var result = locations.GetAncestorIds(l => l.LocationId, l => l.FkParentLocationId, 5);