I am getting this error when I GetById() on an entity and then set the collection of child entities to my new list which comes from the MVC view.
The
I've no idea why the other two answers are so popular!
I believe you were right in assuming the ORM framework should handle it - after all, that is what it promises to deliver. Otherwise your domain model gets corrupted by persistence concerns. NHibernate manages this happily if you setup the cascade settings correctly. In Entity Framework it is also possible, they just expect you to follow better standards when setting up your database model, especially when they have to infer what cascading should be done:
You have to define the parent - child relationship correctly by using an "identifying relationship".
If you do this, Entity Framework knows the child object is identified by the parent, and therefore it must be a "cascade-delete-orphans" situation.
Other than the above, you might need to (from NHibernate experience)
thisParent.ChildItems.Clear();
thisParent.ChildItems.AddRange(modifiedParent.ChildItems);
instead of replacing the list entirely.
UPDATE
@Slauma's comment reminded me that detached entities are another part of the overall problem. To solve that, you can take the approach of using a custom model binder that constructs your models by attempting to load it from the context. This blog post shows an example of what I mean.