Is it possible to remove child from collection and resolve issues on SaveChanges?

Question

We are using Entity Framework Code First with Foreign Key relationships. We investigating on ways on handling removing objects from an entities ICollection in our applicatio

Answer 1

I don't know if the following is a solution for you but Entity Framework supports Identifying Relationships. In such a relationship the foreign key of the child entity (dependent) to the parent (principal) must be part of the (composite) primary key of the child entity. For example - with DbContext data annotations - your model classes have to look like this:

public class Order
{
    [Key]
    public int OrderId { get; set; }

    public ICollection<OrderLine> OrderLines { get; set; }
}

public class OrderLine
{
    [Key, ForeignKey("Order"), Column(Order = 1)]
    public int OrderId { get; set; }

    [Key, Column(Order = 2)]
    public int OrderLineId { get; set; }

    public Order Order { get; set; }
}

You can make the OrderLineId an autogenerated identity if you want. Important is only that the FK to Order is part of the PK.

A code like this for example...

using (var ctx = new MyContext())
{
    var order = ctx.Orders.Include("OrderLines").Single(o => o.OrderId == 1);
    var orderLineToDelete = order.OrderLines
        .FirstOrDefault(ol => ol.OrderLineId == 5);
    if (orderLineToDelete != null)
        order.OrderLines.Remove(orderLineToDelete);

    ctx.SaveChanges();
}

...would indeed delete the orderLineToDelete from the database.

More details are here in section "Considerations for Identifying and Non-identifying Relationships".

Answer 2

As you're finding, if you just remove an Entity from a collection the Entity hangs around attached to the object context and gives you an error when you call SaveChanges(); I've used Domain Events to enable a tidy way of removing the Entity from the object context via a Repository.

I've detailed this approach in my answer to this question.