Question

I follow by entity framework example :

http://msdn.microsoft.com/en-us/library/bb399182.aspx

and I have problem with Identity Columns.

Here is part of code of creating database:

CREATE TABLE [dbo].[Person](
    [PersonID] [int] IDENTITY(1,1) NOT NULL,
    [LastName] [nvarchar](50) NOT NULL,
    [FirstName] [nvarchar](50) NOT NULL,
    [HireDate] [datetime] NULL,
    [EnrollmentDate] [datetime] NULL,
 CONSTRAINT [PK_School.Student] PRIMARY KEY CLUSTERED 
(
    [PersonID] ASC
)WITH (IGNORE_DUP_KEY = OFF) ON [PRIMARY]
) ON [PRIMARY]
END
GO

In VS 2010 I build .edmx and at model I see that Person StoreGeneratedPattern is set to Identity.

But when I want to create Person, by :

Why I must put id, if this column is autoincrement?

EDITŁ

I thought that I found the way to resolve my problem by:

var schoolContext = new SchoolEntities();

            schoolContext.AddToPeople(new Person() { LastName = "Gates", FirstName = "Bil" });

            schoolContext.SaveChanges();

because it added Bill to persons, but…because PersonID is not nullable, and it inserted him with id 0. When I tried add another person the same way of course I get error about primary key 🙂

So still with nothing…

Any ideas ?

Answer 1

I cannot tell you why the EF team chose to do it this way – my only guess would be that the code generation that create the CreatePerson method doesn’t check to see whether or not the ID is an autoincrement and just creates a method that will work in any circumstance – whether the ID is autoincrement or not.

If this really bothers you, you can also benefit from the fact that the generated entity class Person is defined as a partial class, so you can extend it easily. Create a new class file called e.g. PersonEx.cs and extend that partial class:

public partial class Person 
{
    public static Person CreatePerson(string lastName, string firstName)
    {
        return CreatePerson(-1, lastName, firstName);
    }
}

Now, you can easily create your Person entities without specifying an ID, and add them to your data:

 using(SchoolEntities context = new SchoolEntities())
 {
     Person newPerson = Person.CreatePerson("Gates", "Bill");
     context.AddToPeople(newPerson);

     context.SaveChanges();

     int newID = newPerson.PersonID;
 }

It’s not a perfect solution – but it should work just fine (at least it does for me).