Explicity show many-to-many tables in Entity Framework - vb.net

Question

I am attempting to learn Entity Framework to try to move on from Linq to SQL, and my attempt to convert some code over failed on a many-to-many recursive table (a tree structure). I need to do a full table read and prepare the tree in memory because recursing through the database with a lot of queries is too slow.
I have a database with a Projects table, and another table called ProjectsTree. With Linq to SQL, I can get access to the ProjectsTree table, but not with Entity Framework. It puts it in an association in a way that I can't seem to query this table directly.
Here's the code before I attempted to convert Linq to SQL over to Entity Framework, and it worked. Maybe I should stick with Linq to SQL and not learn something new, and if there is no way to do this, I may go backward, or let the two co-exist.
Private Class cProjectTree2
Public Project As PDMVault.Project
Public ChildTree As List(Of cProjectTree2)
End Class
''' <summary>
''' Gets the complete PDM project tree that can be added to a tree node.
''' Each node has the description in the node text field and the primary key in the tag.
''' </summary>
''' <returns></returns>
Public Function GetPDMProjectTree() As TreeNode
' To generate the tree, first read the projects table with a complete table scan, then the ProjectTree with a complete table scan.
' Create a dictionary of objects of type cRecursiveProjectTree, but only the project is set on the first pass, with a reference to it based on its key.
' After the dictionary is built, then link up children to parent using the dictinary.
' Finally, use the created tree to create the node structure for the tree view.
Dim Diag_Stopwatch As New Stopwatch
Dim IDtoTree As New Generic.Dictionary(Of Long, cProjectTree2)
Dim C = New PDMVault.DataContext1
' C.Log = Console.Out
Dim Projects = C.Projects.ToList ' Database list of trees.
''''''''''''''''''''''This is the line that fails. ProjectTrees only shows up as an association, and I can't seem to get access to it for a full table scan
Dim Tree = C.ProjectTrees.ToList ' Database hierarcy of the projects in the previous statement
'''''''''''''''''''''''''''''''''''''''''''''
' Create the dictionary with only the "Project" item set
For Each P In Projects
Dim ProjectTreeEntry As New cProjectTree2
ProjectTreeEntry.ChildTree = New List(Of cProjectTree2)
ProjectTreeEntry.Project = P
IDtoTree.Add(P.ProjectID, ProjectTreeEntry)
Next
' Now that the dictionary has been created, the children can be linked to the parent.
For Each T In Tree
Dim Parent As cProjectTree2 = Nothing
Dim Child As cProjectTree2 = Nothing
IDtoTree.TryGetValue(T.ProjectID, Parent)
IDtoTree.TryGetValue(T.ChildProject, Child)
Parent.ChildTree.Add(Child)
Next
' The tree has been built, create the tree view from the tree (using a little recursion)
Dim GetChildTrees As Func(Of cProjectTree2, TreeNode) =
Function(P As cProjectTree2) As TreeNode
Dim Result As New TreeNode
For Each Child In P.ChildTree.OrderBy(Function(ProjectNode) ProjectNode.Project.Name)
Dim N = GetChildTrees(Child)
If N IsNot Nothing Then
Result.Nodes.Add(N)
End If
Next
Result.Tag = P.Project.ProjectID
Result.Text = P.Project.Name
Return Result
End Function
Dim RootProject As cProjectTree2 = Nothing
If IDtoTree.TryGetValue(1, RootProject) = True Then
Dim N2 As TreeNode = GetChildTrees(RootProject)
Return N2
Else
Return Nothing
End If
End Function

Answer 1

I came very close to going backwards and sticking with LINQ to SQL, but this is a new project and I wanted to learn EF before I had significant code investment. With a little experimenting with Entities Framework, the following handles the recursive tree nicely without me having to get access to the ProjectTrees table.
Public Function GetPDMProjectTree() As TreeNode
Dim Diag_Stopwatch As New Stopwatch
Diag_Stopwatch.Start()
Dim C = New PDMVault.DataContext1
C.Configuration.LazyLoadingEnabled = False ' Necessary for the materialization to work in the next line
Dim MaterializeDatabase = C.Projects.ToList
C.Database.Log = Sub(Log) Debug.Print(Log) ' Verify that only two table scans occurs and that it's not hitting repeatedly
Dim RootProject = (From P In C.Projects Where P.ProjectID = 1).SingleOrDefault
If RootProject Is Nothing Then Return Nothing
Dim GetTree As Func(Of PDMVault.Project, TreeNode) =
Function(P As PDMVault.Project) As TreeNode
Dim Result As New TreeNode
For Each Child In P.Projects1.OrderBy(Function(ProjectNode) ProjectNode.Name)
Result.Nodes.Add(GetTree(Child))
Next
Result.Tag = P.ProjectID
Result.Text = P.Name
Return Result
End Function
If RootProject Is Nothing Then Return Nothing
Debug.Print($"Tree building time={Diag_Stopwatch.ElapsedMilliseconds / 1000:#0.00}")
Return GetTree(RootProject)
End Function
For the SO archives, here is a former method that did it with two trips to the database (one for the initial dictionary, and one for the root) and used an external dictionary before I learned about turning off lazy loading, probably not as optimal as my final solution.
Public Function GetPDMProjectTree2() As TreeNode
Dim Diag_Stopwatch As New Stopwatch
Dim C = New PDMVault.DataContext1
C.Database.Log = Sub(Log) Debug.Print(Log) ' Verify that only one table scan occurs and that it isn't an N+1 problem.
' Force a complete table scan before starting the recursion below, which will come from cached content
Dim ProjectTreeFromDatabase = (From P In C.Projects
Select Project = P,
Children = P.Projects1).ToDictionary(Function(F) F.Project.ProjectID)
Dim GetTree As Func(Of PDMVault.Project, TreeNode) =
Function(P As PDMVault.Project) As TreeNode
Dim Result As New TreeNode
For Each Child In ProjectTreeFromDatabase(P.ProjectID).Children.OrderBy(Function(ProjectNode) ProjectNode.Name)
Dim N = GetTree(Child)
If N IsNot Nothing Then
Result.Nodes.Add(N)
End If
Next
Result.Tag = P.ProjectID
Result.Text = P.Name
Return Result
End Function
Dim RootProject = (From P In C.Projects Where P.ProjectID = 1).SingleOrDefault
If RootProject Is Nothing Then Return Nothing
Return GetTree(RootProject)
End Function
Both solutions prevent repeated trips to the database.