为什么此代码适用于 SQL Server Express 但*不适用于* SQL Server CE？

Question

I get a NullReferenceException when my target provider is SQL Server CE, but with SQL Server Express it works. I get the exception on the return... when I'm trying to access the Comments . Model is very simple and it can be inferred from the code. Thanks for any help. I'm using Entity Framework code-first 4.3 and SQL Server CE 4.0 by the way.

• using the same code with SQL Server Express as provider = runs fine

• using the same code with SQL Server CE as provider = exception error

In addition, I have no problems with SQL Server CE. It works on my machine and I have done single-table operations with Entity Framework code-first and even asp.net authorization. I have never done any nested addition/deletion like this on related entities though so I'm curious as to why this doesn't work..

            db.Persons.Add(new Person()
            {
                FullName = "JC Viray",
                Comments = new List<Comment>(){
                         new Comment(){CommentContent="Hello!"},
                         new Comment(){CommentContent="World!"}
            }
            });

            db.SaveChanges();

            return db.Persons.FirstOrDefault().Comments.FirstOrDefault().CommentContent;

What stumps me also is that in SQL Server Express, if I examine the data, the comments are there.

In SQL Server CE, only the Persons table has data while the Comments table is empty.

EDIT:

If it helps, here is the model:

    public class Person
{
    [Key]
    public int PersonId { get; set; }
    public string FullName { get; set; }

    public virtual ICollection<Comment> Comments { get; set; }
}

public class Comment
{
    [Key]
    public int CommentId { get; set; }
    public string CommentContent { get; set; }

    [ForeignKey("Person")]
    public int PersonId { get; set; }
    public virtual Person Person { get; set; }
}

Answer 1

The problem is that: SQL CE error details are a lot more vague than when using SQL Server Express as provider therefore the confusion and the vague direction.

SQLCE Exception details is something like:

NullException

SQLExpress Exception details is something like:

The model backing the 'MyContext' context has changed since the database was created. Consider using Code First Migrations to update the database (http://go.microsoft.com/fwlink/?LinkId=238269).

The solution is: to sync the models by adding this code:

Database.SetInitializer(new DropCreateDatabaseAlways<MyContext>());