Question

I have been tasked with building a new system that models a virtual filesystem. My client has mandated that this be built using Entity Framework. I have handled node-like data structures before, but never with Entity Framework.

What is the best approach to build a hierarchical class in Entity Framework? By hierarchical, I mean that a class can have a parent of the same type, and can have zero or more children of the same type.

I’m using SQL Server 2008 and Entity Framework 4.0. Should I use the built-in hierarchy data type, or do I go the ParentID route? Suggestions would be most welcome.

Answer 1

I had the same problem. I found that the best way to work with hierarchyid datatype and still be using EF 4.0 is building a view over the the hierarchy table.

Since the view is not updateable I created a delete, insert and add stored procedures and mapped them to the entity mapping in the ORM. This is working really good.

Lets say you have this table:

CREATE TABLE [dbo].[NodeHierarchy]
(
[Node] hierarchyid NOT NULL,
[NodeId] int NOT NULL,
[Level] AS ([Node].[GetLevel]()) PERSISTED,
[Lineage] AS ([Node].[ToString]()) PERSISTED,
[RootNode] AS ([Node].[GetAncestor]([Node].[GetLevel]() - 1)) PERSISTED,
[ParentNode] AS ([Node].[GetAncestor](1)) PERSISTED
)

Now you create this view over it:

CREATE VIEW [dbo].[NodeHierarchyView]
AS
SELECT   ch.NodeId AS [NodeId],
       ch.Node.ToString() AS [Lineage],
 ch.[Level] AS [Level],
 chr.Node.ToString() AS [RootLineage],
 chr.NodeId AS [RootNodeId],
 chp.Node.ToString() As [ParentLineage],
 chp.NodeId AS [ParentNodeId]
FROM     dbo.NodeHierarchy ch
 LEFT OUTER JOIN NodeHierarchy chr ON
      ch.RootNode = chr.Node
 LEFT OUTER JOIN CompanyHierarchy chp ON
      ch.ParentNode = chp.Node

Now I can create an entity in the model over the view and use Linq-to-Entities and get good performace and neat code.

This is the add stored procedure I use:

CREATE PROCEDURE [dbo].[AddNode]
@NodeId int,
@ParentNodeId int
AS
DECLARE @NewNode hierarchyid;
DECLARE @ParnetLineage nvarchar(4000);

SELECT  @ParnetLineage = Lineage
FROM    NodeHierarchy
WHERE   NodeId = @ParentNodeId

IF @ParnetLineage IS NULL
BEGIN
    SET @ParnetLineage = N'/';
END

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
BEGIN TRANSACTION
    SET @NewNode = CAST(@ParnetLineage + CAST(@NodeId AS nvarchar (4000)) + N'/' AS hierarchyid);

    INSERT NodeHierarchy (Node, NodeId)
    VALUES (@NewNode, @NodeId)
COMMIT

SELECT @NodeId AS [NewNodeId]
RETURN 0

I created all the needed indexes and constraints over the tables. In my solution the view is displaying data from other tables and the procedures manipulate these tables as well.

Oded