ASP MVC Blog Database not creating Id number

I have been going through the early stages of a tutorial on youtube http://www.youtube.com/watch?v=WwmUFTWEh6Y

public ActionResult Update(int? id, string title, string body, DateTime dateTime, string tags)
{
    if (!IsAdmin)
    {
        return RedirectToAction("Index");
    }

    Post post = GetPost(id);
    post.Title = title;
    post.Body = body;
    post.DateTime = dateTime;
    post.Tags.Clear();

    tags = tags ?? string.Empty;
    string[] tagNames = tags.Split(new char[] { ' ' }, StringSplitOptions.RemoveEmptyEntries);
    foreach (string tagName in tagNames)
    {
        post.Tags.Add(GetTag(tagName));
    }

    if (!id.HasValue)
    {
        model.AddToPosts(post);
    }
    model.SaveChanges();
    return RedirectToAction("Details", new { id = post.ID });
}

public ActionResult Edit(int? id)
{
    Post post = GetPost(id);
    StringBuilder tagList = new StringBuilder();
    foreach (Tag tag in post.Tags)
    {
        tagList.AppendFormat("{0} ", tag.Name);
    }
    ViewBag.Tags = tagList.ToString();
    return View(post);
}


private Tag GetTag(string tagName)
{
    return model.Tags.Where(x => x.Name == tagName).FirstOrDefault() ?? new Tag() { Name = tagName };
}

private Post GetPost(int? id)
{
    return id.HasValue ? model.Posts.Where(x => x.ID == id).First() : new Post() { ID = -1 };
}

I am getting an aspx error "Cannot insert explicit value for identity column in table 'Posts' when IDENTITY_INSERT is set to OFF." I know this is because it is trying to insert an ID of -1 when the database updates and I dont want that. Should it just create a new ID if im passing in -1? Thats whats happens in the tutorial. I dont know why mine is acting differently.

=========================================================== EDIT The database was created using the database designer not through code, I have looked at the code though and cannot see any Autogenerated property or IsDbGenerated attribute, where on earth would I insert this?

 // <summary>
/// No Metadata Documentation available.
/// </summary>
[EdmEntityTypeAttribute(NamespaceName="CollateralSystems.Models", Name="Post")]
[Serializable()]
[DataContractAttribute(IsReference=true)]
public partial class Post : EntityObject
{
    #region Factory Method

    /// <summary>
    /// Create a new Post object.
    /// </summary>
    /// <param name="id">Initial value of the ID property.</param>
    /// <param name="title">Initial value of the Title property.</param>
    /// <param name="dateTime">Initial value of the DateTime property.</param>
    /// <param name="body">Initial value of the Body property.</param>
    public static Post CreatePost(global::System.Int32 id, global::System.String title, global::System.DateTime dateTime, global::System.String body)
    {
        Post post = new Post();

        post.ID = id;
        post.Title = title;
        post.DateTime = dateTime;
        post.Body = body;
        return post;
    }

    #endregion
    #region Primitive Properties

    /// <summary>
    /// No Metadata Documentation available.
    /// </summary>
    /// 

    [EdmScalarPropertyAttribute(EntityKeyProperty=true, IsNullable=false)]
    [DataMemberAttribute()]
    public global::System.Int32 ID
    {
        get
        {
            return _ID;
        }
        set
        {
            if (_ID != value)
            {
                OnIDChanging(value);
                ReportPropertyChanging("ID");
                _ID = StructuralObject.SetValidValue(value);
                ReportPropertyChanged("ID");
                OnIDChanged();
            }
        }
    }
    private global::System.Int32 _ID;
    partial void OnIDChanging(global::System.Int32 value);
    partial void OnIDChanged();