c# - Can EF automatically delete data that is orphaned, where the parent is not deleted?

Question

For an application using Code First EF 5 beta I have:

public class ParentObject
{
    public int Id {get; set;}
    public virtual List<ChildObject> ChildObjects {get; set;}
    //Other members
}

and

public class ChildObject
{
    public int Id {get; set;}
    public int ParentObjectId {get; set;}
    //Other members
}

The relevant CRUD operations are performed by repositories, where necessary.

In

OnModelCreating(DbModelBuilder modelBuilder)

I have set them up:

modelBuilder.Entity<ParentObject>().HasMany(p => p.ChildObjects)
            .WithOptional()
            .HasForeignKey(c => c.ParentObjectId)
            .WillCascadeOnDelete();

So if a ParentObject is deleted, its ChildObjects are too.

However, if I run:

parentObject.ChildObjects.Clear();
_parentObjectRepository.SaveChanges(); //this repository uses the context

I get the exception:

The operation failed: The relationship could not be changed because one or more of the foreign-key properties is non-nullable. When a change is made to a relationship, the related foreign-key property is set to a null value. If the foreign-key does not support null values, a new relationship must be defined, the foreign-key property must be assigned another non-null value, or the unrelated object must be deleted.

This makes sense as the definition of the entities includes the foreign key constraint which is being broken.

Can I configure the entity to "clear itself up" when it gets orphaned or must I manually remove these ChildObjects from the context (in this case using a ChildObjectRepository).

Answer 1

It is actually supported but only when you use Identifying relation. It works with code first as well. You just need to define complex key for your ChildObject containing both Id and ParentObjectId:

modelBuilder.Entity<ChildObject>()
            .HasKey(c => new {c.Id, c.ParentObjectId});

Because defining such key will remove default convention for auto incremented Id you must redefine it manually:

modelBuilder.Entity<ChildObject>()
            .Property(c => c.Id)
            .HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);

Now calling to parentObject.ChildObjects.Clear() deletes dependent objects.

Btw. your relation mapping should use WithRequired to follow your real classes because if FK is not nullable, it is not optional:

modelBuilder.Entity<ParentObject>().HasMany(p => p.ChildObjects)
            .WithRequired()
            .HasForeignKey(c => c.ParentObjectId)
            .WillCascadeOnDelete();