Using the entity framework to add existing entities to a collection on a newly created entity

问题

I am using the Entity framework to create a new order. The order contains a collection of contacts, a many to many relationship. I want to add a reference to an existing contact on the order on creation of the order. Both Order and Contact a Entity Objects.

 Order order = new Order();

 //set details on order

 Contact contact = new Contact();

 EntityKey contactKey =
                    new EntityKey("OrderDetails.Contact",
                        "contact_id", contact.Key.Id);

 contact.EntityKey = contactKey;
 contact.contact_id = contact.Key.Id;

 order.Contact.Attach(contact);  // throws an exception!

 OrderDetails ordTable = new OrderDetails();
            ordTable.AddToOrder(order);
            int result = orgTable.SaveChanges();

When I go to attach, this exception is thrown:

"Attach is not a valid operation when the source object associated with this related end is in an added, deleted, or detached state. Objects loaded using the NoTracking merge option are always detached."

I know I'm probably missing a step or not fully understanding how the entity framework handles many-to-many relationships.

回答1:

"Attach" is not allowed because you haven't saved the order yet. Calling "Add" tells Entity Framework that you want to insert a new contact. So you are left with only one option. You need to load the contact.

Here's the fastest way to do that:

OrderDetails context = new OrderDetails();
Contact contact = context.GetObjectByKey(new EntityKey("OrderDetails.Contact", "contact_id", existingContactId));
order.Contact.Add(contact);

回答2:

If Order has a property Contact, then you can do:

order.Contact.Add(contact);

I would suggest making the property called Contacts rather than Contact, though.

来源：https://stackoverflow.com/questions/407712/using-the-entity-framework-to-add-existing-entities-to-a-collection-on-a-newly-c

标签

entity-framework

many-to-many