SqlException when creating a new item in Asp.Net MVC 3 app - asp.net

Question

In my "web store" mvc app I want to add items to database. Items table has CreatedBy field and it is a foreign key from User table UserId field. Everything was working before I put the database into the App_Data folder. Now I get the SqlException when trying to create a new Item:
The INSERT statement conflicted with the FOREIGN KEY constraint "FK_Item_contains_User". The conflict occurred in database "C:\USERS\ALEKSEY\REPOS\2\WEBSTORE\WEBSTORE\APP_DATA\WEBSTORE.MDF", table "dbo.Users", column 'UserId'.
Here is the Create method of ItemRepository class:
public Item CreateItem(int productId, Guid userId)
{
var item = new Item
{
ProductId = productId,
CreatedBy = userId,
};
_dataContext.Items.InsertOnSubmit(item);
_dataContext.SubmitChanges(); // in this line the exception occures !
return item;
}
Here is the controller method Create:
[HttpGet]
public ViewResult Create()
{
var p = _productRepository.CreateProduct("", "", 0, "", "", "");
var userId = (Guid)Membership.GetUser().ProviderUserKey;
var item = _itemsRepository.CreateItem(p.ProductId, userId);
// some code
return View(model);
}
Besides, I use Linq to Sql model drag an' drop approach.
Here is the changed web.config connection string part:
<connectionStrings>
<add name="WebStoreConnectionString" connectionString="Data Source=(LocalDB)\v11.0;
AttachDbFilename=|DataDirectory|\WebStore.mdf;Integrated Security=True;Connect Timeout=30"
providerName="System.Data.SqlClient" />
<add name="DefaultConnection" connectionString="Data Source=|DataDirectory|\aspnet.sdf"
providerName="System.Data.SqlServerCe.4.0" />
As I said everything was working before I moved the database to App_Data file. I also tried to remove the dependency between Items and Users tables - the exact same exception.
Any help would be appropriate. Thanks in advance!
Edits:
Ok, now I really broke the dependency between Items and Users tables and no exception occures. But! I have to somehow know who has created each product, so breaking the dependency is not an option. I also tried to remove all code that initializes the CreatedBy field.
Any ideas??
Edits (part 2):
The second comment below gives a great advise! I found that all users that are created are stored now in the aspnet.sdf database!!!
But if I remove the connection string "DeafaultConnection":
<add name="DefaultConnection" connectionString="Data Source=|DataDirectory|\aspnet.sdf"
providerName="System.Data.SqlServerCe.4.0" />
I will get ConfigurationErrorsException:
"The connection name 'DefaultConnection' was not found in the applications
configuration or the connection string is empty."
in the folowing line:
var userId = (Guid)Membership.GetUser().ProviderUserKey;

Answer 1

Ok, as I guessed the issue was in the configuration. Each provider (for some reason) in the connection string had "DefaultConnection". I changed it to "WebStoreConnectionString". And now everything works!
p.s. thanks #w0lf, he pushed the thoughts in the right direction)