How do I delete a child entity from a parent collection with Entity Framework 4?

问题

I'm using Entity Framework 4 and have a one-to-many relationship between a parent and child entity. I'm trying to delete a child using the parent repository by removing it from the parent's children collection:

public virtual void RemoveChild(Child child)
        {
            children.Remove(child);
        }

When I try to save the changes I get the following error:

A relationship from the 'ParentChild' AssociationSet is in the 'Deleted' state. Given multiplicity constraints, a corresponding 'Child' must also in the 'Deleted' state.

Surely I don't have to delete the child entity explicitly using a child repository!

回答1:

It depends on whether you have a cascade in the DB. If you do (and, given your question, you probably should), then this should be automatic. You can read about this here.

回答2:

What I've come across to solve this problem is the follwoing override on the DbContext:

public override int SaveChanges()
{
    Set<Child>()
    .Local
    .Where(r => !Parent.Local.SelectMany(s => s.Children).Contains(r))
    .ToList()
    .ForEach(r => Set<Child>().Remove(r));

    return base.SaveChanges();
}

Take a look here: http://blog.oneunicorn.com/2012/06/02/deleting-orphans-with-entity-framework/

回答3:

In EF4, if the child objects and the parent object are all attached to the ObjectContext, then EF will keep the Foreign Key references and Object References in-sync in both the Parent and the affected Children.

来源：https://stackoverflow.com/questions/2883797/how-do-i-delete-a-child-entity-from-a-parent-collection-with-entity-framework-4