Entity Framework - Clear a Child Collection

Question

I have run into an interesting problem with Entity Framework and based on the code I had to use to tackle it I suspect my solution is less than ideal. I have a 1-to-Many rel

Answer 1

Trick: When setting up the relationship between Parent and Child, you'll HAVE TO create a "composite" key on the child. This way, when you tell the Parent to delete 1 or all of its children, the related records will actually be deleted from the database.

To configure composite key using Fluent API:

modelBuilder.Entity<Child>().HasKey(t => new { t.ParentId, t.ChildId });

Then, to delete the related children:

var parent = _context.Parents.SingleOrDefault(p => p.ParentId == parentId);

var childToRemove = parent.Children.First(); // Change the logic 
parent.Children.Remove(childToRemove);

// you can delete all children if you want 
// parent.Children.Clear();

_context.SaveChanges();

Done!

Answer 2

Clear() removes the reference to the entity, not the entity itself.

If you intend this to be always the same operation, you could handle AssociationChanged:

Entity.Children.AssociationChanged += 
    new CollectionChangeEventHandler(EntityChildrenChanged);
Entity.Children.Clear();            

    private void EntityChildrenChanged(object sender,
        CollectionChangeEventArgs e)
    {
        // Check for a related reference being removed. 
        if (e.Action == CollectionChangeAction.Remove)
        {
            Context.DeleteObject(e.Element);
        }
    }

You can build this in to your entity using a partial class.

Answer 3

You can create Identifying relationship between parent and child entities and EF will delete child entity when you delete it from parent's collection.

    public class Parent
    {
      public int ParentId {get;set;}
      public ICollection<Child> Children {get;set;}
    }

    public class Child
    {          
      public int ChildId {get;set;}
      public int ParentId {get;set;}
    }

Mapping configuration:

    modelBuilder.Entity<Child>().HasKey(x => new { x.ChildId, x.ParentId });
    modelBuilder.Entity<Parent>().HasMany(x => x.Children).WithRequired().HasForeignKey(x => x.ParentId);