Remove an item from Collection using Entity Framework

c# domain-driven-design ef-code-first entity-framework

Question

I am using DDD. I have a class Product which is an aggregate root.

public class Product : IAggregateRoot
{
    public virtual ICollection<Comment> Comments { get; set; }

    public void AddComment(Comment comment)
    {
        Comments.Add(comment);
    }

    public void DeleteComment(Comment comment)
    {
        Comments.Remove(comment);
    }
}

The layer which holds the models doesn't know about EF at all. The problem is that when i call DeleteComment(comment), EF throws exception

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

Even if the element is removed from the collection, EF doesn't delete it. What should i do to fix this without breaking DDD? (I am thinking of making a repository for Comments as well, but is not right)

Code example:

Because i am trying to use DDD, the Product is an aggregate root, and it has a repository IProductRepository. A Comment cannot exists without a product, therefore is a children of Product Aggregate, and Product is responsible for creating and deleting Comments. Comment does not have a Repository.

public class ProductService
{
    public void AddComment(Guid productId, string comment)
    {
        Product product = _productsRepository.First(p => p.Id == productId);
        product.AddComment(new Comment(comment));
    }

    public void RemoveComment(Guid productId, Guid commentId)
    {
        Product product = _productsRepository.First(p => p.Id == productId);
        Comment comment = product.Comments.First(p => p.Id == commentId);
        product.DeleteComment(comment);


        // Here i get the error. I am deleting the comment from Product Comments Collection,
        // but the comment does not have the 'Deleted' state for Entity Framework to delete it

        // However, i can't change the state of the Comment object to 'Deleted' because
        // the Domain Layer does not have any references to Entity Framework (and it shouldn't)

        _uow.Commit(); // UnitOfWork commit method

    }
}
1
9
11/21/2012 7:34:59 AM

Popular Answer

I've seen a lot of people reporting this issue. It's actually quite simple to fix but makes me think there is not enough documentation on how EF is expected to behave in this situation.

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);

// or, you can delete all children 
// parent.Children.Clear();

_context.SaveChanges();

Done!

12
7/9/2014 4:19:37 AM


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