实体框架6多对多

Question

I'm trying to learn to use Entity Framework and am working through the Entity Framework Recipes 6 Book (not even going to try and hide that).
Working on 2-4:
3 tables:
Order: OrderID, OrderDate, (Nav Properties OrderItems)
OrderItem: OrderId, SKU, Count (Nav Properties, Item, Order)
Item: SKU, Description, Price (Nav Properties, OrderItems)

using (var context = new EFRecipesContext()) {
    var order = new Order { OrderId = 1, OrderDate = new DateTime(2010, 1, 18) };
    var item = new Item { SKU = 1729, Description = "Backpack", Price = 29.97M };
    var oi = new OrderItem { Order = order, Item = item, Count = 1 };
    item = new Item { SKU = 2929, Description = "Water Filter", Price = 13.97M };
    oi = new OrderItem { Order = order, Item = item, Count = 3 };
    item = new Item { SKU = 1847, Description = "Camp Stove", Price = 43.99M };
    oi = new OrderItem { Order = order, Item = item, Count = 1 };
    context.Orders.Add(order);
    context.SaveChanges();
}

The only thing that gets added to the database is the order and nothing else, none of the items, etc... I had a problem with the previous example and had to add the "other" items individually but I thought that point was that you could just do the one "Add" and it would add all the objects to the Database?

Thanks in advance!

UPDATE
Ok I made the following changes based on the suggestions below and now it's working

using (var context = new EFRecipesContext()) {
            var order = new Order { OrderId = 1, OrderDate = new DateTime(2010, 1, 18) };
            var item = new Item { SKU = 1729, Description = "Backpack", Price = 29.97M };
            var oi = new OrderItem { Order = order, Item = item, Count = 1 };
            order.OrderItems.Add(oi);   // suggestion from Stackoverflow.

            item = new Item { SKU = 2929, Description = "Water Filter", Price = 13.97M };
            oi = new OrderItem { Order = order, Item = item, Count = 3 };
            order.OrderItems.Add(oi);   // suggestion from Stackoverflow.

            item = new Item { SKU = 1847, Description = "Camp Stove", Price = 43.99M };
            oi = new OrderItem { Order = order, Item = item, Count = 1 };
            order.OrderItems.Add(oi);   // suggestion from Stackoverflow.

            context.Orders.Add(order);
            context.SaveChanges();
        }

At least now I know what to change in their code going forward to get it working. Thanks!!

Answer 1

You never actually add the OrderItems to the DbContext collection.

There is also a problem with your code, which means you are overwriting your items and oi variable values multiple times.

The below code makes some assumptions, since you haven't supplied more of your code, but should be easy to adapt.

using (var context = new EFRecipesContext()) {
    var order = new Order { OrderId = 1, OrderDate = new DateTime(2010, 1, 18) };
    context.Orders.Add(order);
    var item1 = new Item { SKU = 1729, Description = "Backpack", Price = 29.97M };
    var oi1 = new OrderItem { Order = order, Item = item, Count = 1 };
    context.OrderItems.Add(oi1);

    var item2 = new Item { SKU = 2929, Description = "Water Filter", Price = 13.97M };
    var oi2 = new OrderItem { Order = order, Item = item, Count = 3 };
    context.OrderItems.Add(oi2);

    var item3 = new Item { SKU = 1847, Description = "Camp Stove", Price = 43.99M };
    var oi3 = new OrderItem { Order = order, Item = item, Count = 1 };
    context.OrderItems.Add(oi3);

    context.SaveChanges();
}

Unless you are using the variables again, you could do the definition of the objects inline like this aswell, which is slightly less repetitive:

    context.OrderItems.Add(new OrderItem { 
        Order = order, 
        Item = new Item { 
            SKU = 1729, 
            Description = "Backpack", 
            Price = 29.97M 
        }, 
        Count = 1 
    });

Answer 2

This is not really about many to many, but about the way how EF discovers related data.

You need to explicitely add the entry to the database, that is actually referencing the related data. In your case, this is oi , so if you add it

context.OrderItems.Add(oi);
context.SaveChanges();

the related io.Order and io.Item will be considered by EF. But EF has no way of knowing about this relation when you only provide order without providing navigation information from order to io .

Another way of solving the issue (instead of adding io explicitely) would be to update order.OrderItems before saving:

oi = new OrderItem { Order = order, Item = item, Count = 1 };
order.OrderItems.Add(oi);
context.Orders.Add(order);
context.SaveChanges();

Answer 3

You must add an entry to the database to be invoked on appropriate data. In this case, it's the OI.