Getting Entity Framework to cascade-delete when re-attaching modified entity - asp.net-mvc

Question

I'm developing a website using MVC4 with EF code first approach.
I have some problems with deleting the children in entities with one to many relations.
Edit to clearify: In my Edit view, I add/remove/update existing childen in the childrens collection on the parent, add/remove is done using javascript. When I receive the updated parent in the post request in the controller method, I want to sync/update the parent and child entities in the database.
The parent object is in a detached state when updated in the view. So, when I attach the parent again, I want it to do all the updates that have been done during detached state.
The entity relations are set up so that when removing a child entity from the parent collection, the child entity is also deleted from the child table (cascade delete sort of?), and this works when in attached state.
However, when attching the parent and saving the changes, only added/updated children are added/modified in the database. But removed children from the parent collection are not deleted in the database (which I want them to be).
How can this be solved??
The entities are:
class Parent
{
public virtual ICollection<Child> Children { get; set; }
}
class Child
{
public string Text { get; set; }
}
This works and will remove child from database:
void RemoveChildFromCollection()
{
// get the first parent and remove the first child in collection
var context = new DatabaseContext();
var parent = context.Parents.First();
parent.Children.Remove(parent.Children.First());
context.SaveChanges();
}
ControllerMethod: This does not work as above, removed children are not removed from the childrens table
public ActionResult Edit(Parent parent)
{
var context = new DatabaseContext();
context.Entry(parent).State = System.Data.EntityState.Modified;
context.SaveChanges();
return View();
}
The modelbuilder are setup to delete child entity from child table when removing them from parent collection
// Use Identifying relation. Define complex key for ChildObject containing both Id and
ParentObjectId
modelBuilder.Entity<Child>()
.HasKey(c => new {c.ChildID, c.ParentID});
// Because defining such key will remove default convention for auto incremented Id you must redefine it manually
modelBuilder.Entity<Child>()
.Property(c => c.ChildID)
.HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
// Set cascade delete
modelBuilder.Entity<Parent>()
.HasMany(p => p.Children)
.WithRequired()
.HasForeignKey(c => c.ParentID)
.WillCascadeOnDelete();

Answer 1

Cascade delete only deletes child entities, when the parent entity is also removed, as you've noted, not when you sever the relationship.
You can override your SaveChanges() in your Context to clean up your orphaned Child entities like this:
public override int SaveChanges()
{
Children
.Local
.Where(c => c.Parent == null)
.ToList()
.ForEach(child => Children.Remove(child));
return base.SaveChanges();
}
This blog post has more info on handling orphaned entities.

Answer 2

Your entities - parent and children - are in a detached state when you do the modification in the view. So, EF couldn't track those changes. When you attach the object graph to the context - by setting the parent's state to Modified - EF takes this attached object graph as the current state and doesn't know anything about the removal of the child that happened during the detached phase in the view.
To solve the problem you must load the current object graph - parent including children - from the database, compare it with the object graph from your view and merge the changes into the loaded graph. Then save the changes. There can be several possible changes:
The parent's scalar properties could have been changed
The children's scalar properties can been changed
A new child could have been added to the Children collection
A child could have been removed from the Children collection
Your current code - setting the parent's state to Modified - will only handle the first case correctly but not the other three cases.
To deal with all four cases you need to follow the procedure described above. An example how to do that is shown here (see the Edit section in that answer).
The code in your Edit post action would then look like this:
public ActionResult Edit(Parent parent)
{
using (var context = new DatabaseContext())
{
var parentInDb = context.Parents
.Include(p => p.Children)
.Single(p => p.ParentId == parent.ParentId);
context.Entry(parentInDb).CurrentValues.SetValues(parent);
foreach (var childInDb in parentInDb.Children.ToList())
if (!parent.Children.Any(c =>
c.ChildId == childInDb.ChildId &&
c.ParentId == childInDb.ParentId)) // or == parent.ParentId
context.Children.Remove(childInDb);
// here
// parentInDb.Children.Remove(childInDb);
// should work too because you have an identifying relationship
foreach (var child in parent.Children)
{
var childInDb = parentInDb.Children.SingleOrDefault(c =>
c.ChildId == child.ChildId &&
c.ParentId == child.ParentId); // or == parent.ParentId
if (childInDb != null)
context.Entry(childInDb).CurrentValues.SetValues(child);
else
parentInDb.Children.Add(child);
}
context.SaveChanges();
return View();
}
}