Best way to remove items which are not anymore in collection

Question

I have a classic Order item in my database:

public partial class ORDERS
{
    public ORDERS()
    {
        this.ORDER_DETAIL = new HashSet<ORDER_DETAIL>();
    }

    public int ORDER_IDE { get; set; }
    public string ORDER_STATE { get; set; }
    public decimal ORDER_TOTAL { get; set; }
    public decimal ORDER_TAXES { get; set; }
    public decimal ORDER_SHIPPING_COST { get; set; }
    public decimal ORDER_HANDLING_COST { get; set; }
    public Nullable<System.DateTime> ORDER_SHIPPING_DATE { get; set; }
    public string ORDER_BILLING_NAME { get; set; }
    public string ORDER_BILLING_ADDRESS { get; set; }
    public string ORDER_BILLING_CITY { get; set; }
    public string ORDER_BILLING_REGION { get; set; }
    public string ORDER_BILLING_COUNTRY { get; set; }
    public string ORDER_BILLING_POSTAL_CODE { get; set; }
    public string ORDER_SHIPPING_NAME { get; set; }
    public string ORDER_SHIPPING_ADDRESS { get; set; }
    public string ORDER_SHIPPING_CITY { get; set; }
    public string ORDER_SHIPPING_REGION { get; set; }
    public string ORDER_SHIPPING_COUNTRY { get; set; }
    public string ORDER_SHIPPING_POSTAL_CODE { get; set; }
    public string ORDER_COMMENT { get; set; }
    public decimal ORDER_DETAIL_AMOUNT { get; set; }
    public string ORDER_DESCRIPTION { get; set; }
    public decimal ORDER_DISCOUNT { get; set; }

    public virtual ICollection<ORDER_DETAIL> ORDER_DETAIL { get; set; }
}

As you can see, this items has a collection of ORDER_DETAIL . In my project I want to save the modifications made to the order and keep only the current order details . So I am doing this:

public void SaveOrderModifications(ORDERS _orderToReceive)
{
    using (mDb = new DatabaseEntity())
    {
        mDb.Database.Connection.Open();

        var orderQry = from o in mDb.ORDERS
                       where o.ORDER_IDE == _orderToReceive.mOrderID
                       select o;

        ORDERS originalOrder = orderQry.FirstOrDefault();

        if (originalOrder == null)
        {
            throw new Exception("Invalid operation");
        }

        mDb.Entry(originalOrder).CurrentValues.SetValues(_orderToReceive);

        mDb.SaveChanges();
    }
}

So if my original order had 3 items, and my new order has 8, and from this order 2 of the original order were dumped, what do I need to do to effectively only keep the 8 new items? Do I need to iterate through all of them to see which ones are there, and which one aren't there anymore?

EDIT

I have found a solution, which is not elegant and consumes a bit of process:

foreach (var orderDetail in originalOrder.ORDER_DETAIL.ToList())
{
    mDb.ORDER_DETAIL.Remove(orderDetail);

    mDb.SaveChanges();
}

foreach (var orderDetail in orderToSave.ORDER_DETAIL)
{
    mDb.ORDER_DETAIL.Add(orderDetail);

    mDb.SaveChanges();
}

it implies that I flush all the older ORDER_DETAIL object before adding the new one, but I'm still looking for a more elegant / better way of doing things.

Answer 1

Typically I do it the same way you are doing it, but I check to see if the item is on the new one and only add and remove the changed items. It adds some elegance because you can use a Linq expression.

Something to the effect of:

foreach (var orderDetail in originalOrder.ORDER_DETAIL.Where(d => !newOrder.ORDER_DETAIL.Contains(d)).ToList())
{
    mDb.ORDER_DETAIL.Remove(orderDetail);

    mDb.SaveChanges();
}