c# - EF - 导航属性为空，即使在重新加载实体之后，但在程序重新启动时有效

Question

编辑

我做了一些测试，发现 Item 导航属性仅在处理上下文/创建新上下文时才有效。

        DataContext context = new DataContext();
        Order ord = context.Orders.FirstOrDefault();
        ord.OrderItem.Add(new OrderItem() { ItemId = 8, Quantity = 2, DateCreated = DateTime.Now });
        // At this point, both Order and Item navigation property of the OrderItem are null

        context.SaveChanges();
        // After saving the changes, the Order navigation property of the OrderItem is no longer null, but points to the order. However, the Item navigation property is still null.

        ord = context.Orders.FirstOrDefault();
        // After retrieving the order from the context once again, Item is still null.

        context.Dispose();
        context = new DataContext();
        ord = context.Orders.FirstOrDefault();
        // After disposing of the context and creating a new one, then reloading the order, both Order and Item navigation props are not null anymore

有人能给我解释一下吗？

编辑结束

在我的程序中，用户有一个订单列表，他可以向其中添加新订单。用户还可以向订单中添加订单项，但由于 OrderItem -> Item 导航属性为 null，即使在保存并重新加载订单后，这在 atm 上也无法正常工作。

OrderItem
---------
OrderId
ItemId
Quantity
DateCreated
---------
Item <- navigation property
Order

当用户更改订单并按下保存按钮时，程序从 View 中获取数据，更新 activeOrder 并将其发送到 orderModel，将其添加为新订单或更新现有订单。

GetOrderDataFromView() ..

        ...
        // Check for existing item, update if match found, add new item if not
        foreach (ItemViewObject oi in orderItems)
        {
            var existingOrderItem = activeOrder.OrderItem.Where(o => o.ItemId == oi.ItemId).SingleOrDefault();

            if (existingOrderItem != null)
            {
                existingOrderItem.Quantity = oi.Quantity;
            }
            else
            {
                activeOrder.OrderItem.Add(new OrderItem()
                {
                    ItemId = oi.ItemId,
                    Quantity = oi.Quantity,
                    DateCreated = DateTime.Now
                });
            }

然后，在 orderModel 类中 ...

    public void AddOrUpdate(Order order)
    {
        if (order.Id == 0)
        {
            context.Orders.Add(order);
        }

        context.SaveChanges();
    }

然后更新订单表，触发一个事件，该事件触发 OrderSelectionChanged() 方法。此时，通过从 orderModel 中检索它来重新加载订单(返回 context.Orders.Where...)...

        // Get values from selected order and populate controls
        if (view.OrderTable.SelectedRows.Count != 0)
        {
            OrderViewObject ovm = (OrderViewObject)view.OrderTable.SelectedRows[0].DataBoundItem;
            activeOrder = orderModel.GetById(ovm.OrderId);

            PopulateOrderItemTableControl();

当调用 PopulateOrderItemTableControl() 方法时，我开始遇到麻烦..

        foreach (OrderItem oi in activeOrder.OrderItem)
        {
            orderItems.Add(new ItemViewObject(oi));
        }

因为在创建新的 ItemViewObject 时，我需要通过其导航属性获取 orderItem 的项目。但是，orderItem.Item 导航属性为 null，我在这里遇到异常 ...

    public ItemViewObject(OrderItem orderItem)
    {
        dateCreated = orderItem.DateCreated;

        // Retrieve latest item details, with effective date older or equal to the creation date of the order item
        var details = orderItem.Item.ItemDetails.Where(i => i.DateEffective  <= dateCreated)
                                                        .OrderByDescending(i => i.DateEffective)
                                                        .FirstOrDefault();

然后我必须重新启动程序，但在这样做之后，订单项加载得非常好。因此，这只会在向订单添加新订单项、保存订单、然后重新加载订单并尝试显示其订单项时发生。

此外，这只会发生在之前未添加到订单中的新商品上。因此，如果我创建一个新项目并将其添加到订单中，单击保存按钮，我将得到一个空异常并需要重新启动。重新启动后，项目加载正常。如果我随后删除订单项目、保存订单、重新添加项目并再次保存，它不会崩溃。

希望这对某人有意义。

干杯!

注意:我先用模型。

Answer 1

该行为实际上是预期的并且有解释:

DataContext context = new DataContext();
Order ord = context.Orders.FirstOrDefault();
ord.OrderItem.Add(new OrderItem() {
    ItemId = 8, Quantity = 2, DateCreated = DateTime.Now });
// At this point, both Order and Item navigation property of the OrderItem
// are null
// Explanation: That's clear because you don't set Order and Item property

context.SaveChanges();
// After saving the changes, the Order navigation property of the OrderItem
// is no longer null, but points to the order. However, the Item navigation
// property is still null.
// Explanation: SaveChanges internally fixes relationships with objects that are
// attached to the context. Order is attached, Item is not. That's why only
// the Order property is set

ord = context.Orders.FirstOrDefault();
// After retrieving the order from the context once again, Item is still null.
// Explanation: Because the Order is already attached a new query won't replace
// it. The entity remains the same as before.

context.Dispose();
context = new DataContext();
ord = context.Orders.FirstOrDefault();
// After disposing of the context and creating a new one, then reloading the
// order, both Order and Item navigation props are not null anymore
// Explanation: In a new context the Order will be loaded as a proxy. So, now
// lazy loading will work and load the Item property

这里的关键问题是您使用 new 运算符创建新的 OrderItem:

ord.OrderItem.Add(new OrderItem() {
    ItemId = 8, Quantity = 2, DateCreated = DateTime.Now });

这意味着它不是能够通过延迟加载来加载导航属性的动态代理。为了手动创建代理，您应该使用:

var newOrderItem = context.OrderItems.Create();
newOrderItem.ItemId = 8;
newOrderItem.Quantity = 2;
newOrderItem.DateCreated = DateTime.Now;

ord.OrderItem.Add(newOrderItem);

在 context.SaveChanges() 之后，您现在应该能够通过延迟加载加载 Item 属性(基于外键值 ItemId).不需要使用 context.Orders.FirstOrDefault() 来“重新加载”订单，也不需要使用新的上下文。