Error in saving related objects in Entity Framework - wpf

Question

I have a structure like this
DRDLines:
ID
DrawingRevisionID
DrawingRevision:
ID
Name
They're related in a one-to-many relationship.
In this code example
DRDLine line;
using (var db = new AMPX_DCEntities())
{
line = db.DRDLines.Single(p => p.ID == 1);
System.Console.WriteLine(line.DrawingRevision.ID);
}
using (var db = new AMPX_DCEntities())
{
var id = 12;
line.DrawingRevisionID = id;
}
using (var db = new AMPX_DCEntities())
{
db.Entry(line).State = System.Data.Entity.EntityState.Modified;
db.SaveChanges();
}
I get this error
A referential integrity constraint violation occurred: The property value(s) of 'DrawingRevision.ID' on one end of a relationship do not match the property value(s) of 'DRDLine.DrawingRevisionID' on the other end.
What I've found: it doesn't update relations in DRDLines inside DrawingRevision
Debugging I see:
line.DrawingRevision.DRDLines[0].ID != line.DrawingRevisionID
If I remove line
System.Console.WriteLine(line.DrawingRevision.ID);
or write it like this
System.Console.WriteLine(line.DrawingRevisionID);
everything goes without errors. But I need that line to be used.
So, how can I fix that?

Answer 1

My guess is that the problem is caused by repeatedly creating a new context and then disposing it. When you set the DrawingRevisionID here
using (var db = new AMPX_DCEntities())
{
var id = 12;
line.DrawingRevisionID = id;
}
line is detached from the dbcontext from which it was retreived, but isn't attached to the new DbContext you've created, hence EF won't wire up the relationships when you change the ID.
You could attach the line object back to the context before changing the ID
db.DRDLines.Attach(line);
That will change both the IDs (although you could just change the other ID manually). Since that context is then disposed, you may need to set the EntityState to Modified for the DrawingRevision (or at least of the ID property) in the last DbContext session.
Also, I would add an Include to the original query to eagerly load the DrawingRevision. At the moment its only loaded when you query the ID on the System.Console line, hence why the behaviour is different. This also causes an extra trip to the database. Putting it into an include will be more efficient and more predictable.