Entity Framework - Add an item to a list more than once - entity-framework

Question

I am creating an Entity Framwork Code First app and am running into a problem when trying to add an entity to a list more than once.
I have the following two classes, which reference each other for a many-to-many relationship.
public class Order
{
public virtual List<OrderItem> OrderItems { get; set; }
}
public class OrderItem
{
public virtual List<Order> Orders{ get; set; }
}
This creates a the following three tables in my database:
Orders
- OrderId (PK, int)
.
OrderItems
- OrderItemId (PK, int)
.
OrderOrderItems
- Order_OrderId (PK,FK,int)
- OrderItem_OrderItemId (PK,FK,int)
In code, I wish to do the following:
private void AddOrderItemsTest
{
OrderItem orderItem = GetOrderItem(); // gets an existing order item from the DB
var order = new Order();
order.OrderItems.Add(orderItem);
order.OrderItems.Add(orderItem); // add the order item to the list a second time
context.Orders.Add(order);
}
When this gets persisted to the database, only a single orderItem entity is added to the list. We see in the table OrderOrderItems that OrderItem_OrderItemId is a PK and therefore must be unique. This means that EF has designed the tables in a way that won't allow more than one orderItem of the same type to be added to the list more than once.
Is there a Data Annotation that I can add to tell EF to allow me to add more than one item of the same type to the list?

Answer 1

I believe that it is not correct way of handling item count (of same item) in your order. Do you really want 10000s duplicated entries loaded into your OrderItems collection? I guess not.
You will need different primary key on OrderOrderItems table, hence suggest to introduce new entity that will contain OrderItem amount per Order:
public class OrderItemDetails
{
public int OrderItemDetailsId { get; set; }
public int OrderId { get; set; }
public int OrderItemId { get; set; }
public int Amount{ get; set; }
public virtual Order Order { get; set; }
public virtual OrderItem OrderItem { get; set; }
}
public class Order
{
public virtual List<OrderItemDetails> OrderItemDetails { get; set; }
}
public class OrderItem
{
public virtual List<OrderItemDetails> OrderItemDetails { get; set; }
}
And if you not happy with introducing Amount and still want to have duplicated entries per each item instance that will be absolutely fine because primary key of your many to many relation will be not combination of OrderId and OrderItemId but OrderItemDetailsId.