Entity Framework EntityKey / Foreign Key problem

Question

As the result of a form post, I'm trying to save a new Brand record. In my view, Gender is a dropdown, returning an Integer, which is populated from ViewData("gender")

I've setup my link as follows:

gID = CInt(Request.Form("Gender"))
Brand.GenderReference.EntityKey = New EntityKey("DB_ENTITIES.Gender", "Id", gID)
TryUpdateModel(Brand)
DB.SaveChanges()

Which results in the following error.

Entities in 'DB_ENTITIES.Brand' participate in the 'FK_Brand_Gender' relationship. 0 related 'Gender' were found. 1 'Gender' is expected.

Could someone explain the parameters in plain english to me. I've also tried DB.Gender as the first parameter but no joy.

Solution 1

Rather than creating an EntityKey create a stub Gender object (sorry I'm not a VB guy so in C#):

Gender g = new Gender{ID = Int32.Parse(Request.Form("Gender"))};

Then you attach the Gender to the appropriate EntitySet (the name of property on DB that you get Gender entities from is the string below):

DB.AttachTo("Genders",g);

This puts the database in the state where the Gender is in the ObjectContext in the unchanged state without a database query. Now you can build a relationship as per normal

brand.Gender = g;
DB.AddToBrand(brand);
DB.SaveChanges();

That is all there is to it. No need to muck around with EntityKeys

Hope this helps

Alex

Solution 2

What you need is an instance of both Brand and Gender and then set Brand.Gender to the instance of the Gender. Then you can save without problems.

Comments

[Squiggs.]

Does that not involve a round trip to my DB? Surely the View is posting back an ID that can be tied up.

[Squiggs.]

Thanks for the response Alex - getting this now: An object with the same key already exists in the ObjectStateManager. The ObjectStateManager cannot track multiple objects with the same key.

[Jonathan van de Veen]

It depends, if the objects are already in the context, it does not require a trip to the db. Alex James code is good to use (it pretty much does as I wrote).

[Picflight]

Paul, how did you solve the 'multiple objects with the same key' issue? I am getting the same.

[Alex James]

Before attaching something to the context you should check the ObjectStateManager using LINQ to Objects to check that the same object isn't already there.