Entity Framework 6 Navigation Property Collections - vb.net

Question

I am developing a database first application in VB.NET using entity framework version 6.1.1 and sql server 2008. I have some pure join tables that link many to many relationships between two tables. My entities are untracked.
Here are basic examples of the classes (generated from the EF tt files) that I am using:
Public Class Part
Public Property id As Long
Public Property name As String
Public Overridable Property CarModels As ICollection(Of CarModel) = New HashSet(Of CarModel)
End Class
Public Class CarModel
Public Property id As Long
Public Property name As String
Public Overridable Property Parts As ICollection(Of Part) = New HashSet(Of Part)
End Class
When I am updating fields for an entity, I set the values, and then include code like this:
obj.Name = "New Name"
context.Entry(obj).State = EntityState.Modified
context.SaveChanges
That will save the Name value to the database as I expect. My problem is trying to add a new CarModel to an existing part, or remove an existing CarModel from a part. I've tried several things but have not found a solution. Here's an example of my code:
Dim p As Part = context.Parts.where(Function(it) it.id.equals(1)).first 'Part I am working with
Dim c As CarModel = context.CarModels.where(Function(it) it.id.equals(1)).first 'Car Model I want to associate to the part
p.CarModels.Add(c) 'Add the Car Model to the part collection
context.Entry(p).State = EntityState.Modified
context.SaveChanges
No error is thrown. When I am debugging the CarModel is Added to the Part.CarModel collection. However, the changes are not committed to the database. If I add a NEW Part and use similar code it work, but I cannot add or remove from an existing collection and get it to commit to the database.

Answer 1

I haven't used VB in 6 years, so this might not be exactly right, but this will give you a general idea on how it works.
Dim p As Part = context.Parts.where(Function(it) it.id.equals(1)).first 'Part I am working with
Dim c As CarModel = context.CarModels.where(Function(it) it.id.equals(1)).first 'Car Model I want to associate to the part
Dim newPart As Part = New Part()
newPart.id = p.id
newPart.name = p.name
newPart.CarModels = c
context.Add(p)
context.SaveChanges()

Answer 2

I took a look at the context itself, and this line was in the constructor of the context:
Configuration.AutoDetectChangesEnabled = False
That is what was causing my particular problem. I read somewhere (after I found that line) that it is recommended to not turn set AutoDetectChangesEnabled to false unless there is a very long running process, and in that case to set it back to true after the process had completed. Removing that line from the context constructor solved my problem.