使用Linq插入SQL的奇怪错误

Question

I'm having a strange problem inserting a row using Linq To SQL, I'm being asked to write a program without use Entity Framework, so I'm figuring how to make this and I'm following a Linq to SQL tutorial.

When I try to insert a row I get this exception:

System.Data.SqlServerCe.SqlCeException: No se puede modificar la columna. [ Column name = id ]

Is Linq is trying to modify a column?, this is my entity, when I remove the Association field I'm able to persist without problem.

[Table(Name="Usuario")]
public class Usuario
{
    [Column(IsPrimaryKey = true, IsDbGenerated = true, AutoSync = AutoSync.OnInsert,Name="id")]
    public int id { get; set; }
    [Column]
    public string nombre { get; set; }
    [Column]
    public string direccion { get; set; }
    [Column]
    public string telefono { get; set; }
    [Column]
    public string codigoPostal { get; set; }

    private EntityRef<Ciudad> _ciudad;

    [Association (Storage="_ciudad",ThisKey="ciudad",OtherKey="id")]
    public Ciudad ciudad {
        get {return this._ciudad.Entity;}
        set { this._ciudad.Entity = value; }
    }

    private EntityRef<TipoUsuario> _tipoUsuario;

    [Association (Storage="_tipoUsuario",ThisKey="tipoUsuario",OtherKey="id")]
    public  TipoUsuario tipoUsuario {
        get {return this._tipoUsuario.Entity;}
        set { this._tipoUsuario.Entity = value; }
    }
}

Update:

Here is when I'm trying to persist:

public override void agregar(Usuario obj)
{
    System.Data.Linq.Table<Usuario> usrs = context.GetTable<Usuario>();
    usrs.InsertOnSubmit(obj);

    try
    {
       context.SubmitChanges();
    }
    catch (Exception e)
    {
    }
}

And this is my controller :

public class UsuarioController : Controller
{
    UsuarioRepository dao = new UsuarioRepository();

    ...

    [HttpPost]
    public ActionResult Create(Usuario u)
    {
        //try
        //{
        dao.agregar(u);
        return RedirectToAction("Index");
        //}
        //catch
        //{
        //    return View();
        //}
    }
   ...
}

Answer 1

The "Id" column is both set to PrimaryKey and DbGenerated. Probably before inserting the "obj" object somehow you are modifying the value of Id which causes this exception.

Answer 2

Finally I made this work

The problem was my Storage, in fact, it causes a stackoverflow exception because is pointing to itself.

So I had to make some changes, renaming my FK columns on "Usuario" table and adding this column to my model, then set the "thiskey" and "otherkey" values and adding "isforeignkey=true".

This is my final model for "usuario" :

namespace Project.Models
{
    [Table(Name="Usuario")]
    public class Usuario
    {
        [Column(IsPrimaryKey = true, IsDbGenerated = true)]
        public int id { get; set; }
        [Column]
        public string nombre { get; set; }
        [Column]
        public string direccion { get; set; }
        [Column]
        public string telefono { get; set; }
        [Column]
        public string codigoPostal { get; set; }
        [Column]
        public Int32 tipoUsuarioId { get; set; }
        [Column]
        public Int32 ciudadId { get; set; }


        private EntityRef<Ciudad> _ciudad;
        [Association(Storage = "_ciudad", Name = "FK_USUARIO_CIUDAD", OtherKey = "id", ThisKey = "ciudadId", IsForeignKey = true)]
        public Ciudad ciudad {
            get {return this._ciudad.Entity;}
            set { this._ciudad.Entity = value; }
        }

        private EntityRef<TipoUsuario> _tipoUsuario = new EntityRef<TipoUsuario>();
        [Association (Storage="_tipoUsuario",Name="FK_USUARIO_TIPOUSUARIO",OtherKey="id",ThisKey="tipoUsuarioId",IsForeignKey=true)]
        public  TipoUsuario tipoUsuario { 
            get {return this._tipoUsuario.Entity;}
            set { this._tipoUsuario.Entity = value; }
        }

    }

Thanks to @aminexplo for your help :)