Getting Entity Framework to cascade-delete when re-attaching modified entity

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();

I have added a code snippet to my answer below to make it clearer. — Slauma, Nov 09 '12 at 14:22

score 2 · Answer 1 · answered Nov 08 '12 at 16:35

2

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.

answered Nov 08 '12 at 16:35

Mark Oreta

10,346
1
33
36

Hm, does this really work if the removal of a child happened in a detached state? There was no EF context available in the view that could have set the child's `Parent` to `null`. In an attached state he doesn't have the problem of orphaned records because the parent-child relationship is an identifying relationship, i.e. it removes orphaned children automatically. – Slauma Nov 08 '12 at 16:46
Is it in a detached state? His question says: "However, when a child is removed from the parent collection in the view, EF does not delete the child..." this implies that the child was attached, then detached while in the view. I guess the real question is, as your answer said - how exactly is he removing the child. It might actually be easier, if in his controller actions he deletes the child, rather than simply removing. – Mark Oreta Nov 08 '12 at 17:28
I was actually *guessing* (and I might be wrong) that he loads the original graph in a GET request, passes it to the view, some - maybe - Javascript removes a child, the view gets posted back and a detached object graph is created by the MVC model binder (`Edit(Parent parent)`) that now contains one child less. But which one? The post action doesn't know what to delete. If he posts the full original collection back with perhaps a `Deleted` flag in one the children then yes, he could just delete that one in the controller. – Slauma Nov 08 '12 at 18:13
@Slauma, you ar spot on! I update the child collection in the view using some javasctipt, but don't know what updates accoured when I receive the updated parent in the post request. I will try to clearify. – Anders Nov 09 '12 at 06:12

score 1 · Answer 2 · edited May 23 '17 at 10:24

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();
    }
}

Getting Entity Framework to cascade-delete when re-attaching modified entity

2 Answers2