EF Model with collection and navigation to a particular element - entity-framework

Question

How to configure a EF6 migration with a model class having?
A collection o items
A navigation property to one particular item
public class MyModel
{
[Key]
public int Id { get; set; }
// My collection of elements
public virtual ICollection<MyCollectionElement> MyCollection { get; set; }
// Optional navigation to a particular element from the collection
[ForeignKey("CurrentElement")]
public int? CurrentElementId { get; set; }
public virtual MyCollectionElement CurrentElement { get; set; }
}
public class MyCollectionElement
{
[Key]
public int Id { get; set; }
// Required navigation to MyClass
[ForeignKey("MyModel")]
public int MyModelID { get; set; }
public virtual MyModel Model { get; set; }
}
Configuration
modelBuilder.Entity<MyModel>()
.HasMany(x => x.MyCollection)
.WithRequired(x => x.Model)
.HasForeignKey(x => x.MyModelID)
.WillCascadadeOnDelete(false);
Throws several errors on Update-Database, like
Unable to determine a valid ordering for dependent operations.
I would like a solution which doesn't involve a boolean IsCurrent in MyCollectionElement to make another query later and find which element is the current; instead, I would like to store the current element's id with my model, like exposed.
Also, I don't mind making int CurrentElementId non nullable (required) if it's easier.
Thanks.

Answer 1

This chicken-and-egg problem always looms when there are circular relationships. The error...
Unable to determine a valid ordering for dependent operations.
...is not thrown when the database is created. The database can be created just fine. It occurs when you try to insert a MyModel record and a MyCollectionElement referring to one another in the same unit of work. In the Seed method you probably have something like
var element = new MyCollectionElement();
var model = new MyModel();
model.MyCollection.Add(element);
model.CurrentElement = element;
The statement model.MyCollection.Add(element); requires model to be inserted first, so element can refer to it in its foreign key. But model.CurrentElement = element; requires element to be inserted first.
You can only avoid this situation by calling SaveChanges twice, and wrapping everything in a TransactionScope if you want the assignments to be transactional:
using(var ts = new TransactionScope())
{
using(var db = new MyContext()
{
var element = new MyCollectionElement();
var model = new MyModel();
model.MyCollection.Add(element);
db.MyModels.Add(model);
db.SaveChanges();
model.CurrentElement = element;
db.SaveChanges();
}
ts.Complete();
}
This also means that int CurrentElementId should remain nullable.