使用LINQ的多对多映射

我想在C_中,在多对多关系中执行LINQ到SQL的映射,但在这种情况下,数据不是必需的。
清楚:
我有一个新闻网站/博客,还有一个叫做 Posts . 一个博客可以同时关联多个类别,因此有一个名为 CategoriesPosts 与外国钥匙的联系 帖子 表与 Categories 表。我用一个身份主键和一个 id 每个字段中的字段,如果在本例中很重要。
在C中,我为每个表定义了一个类,尽可能明确地定义了每个字段。这个 Post 以及 Category 上课,有一个 EntitySet 链接到 CategoryPost 对象,以及 分类帖子 班级有2人 EntityRef 要链接到彼此类型的2个对象的成员。

问题是,一个职位可能与任何类别相关,也可能与某个类别相关,或者不相关。我找不到方法 EntitySet<CategoryPost?> 或者类似的。

因此,当我添加第一篇文章时,没有一条SQL语句,一切都很顺利。此外,这篇文章也出现在输出中。当我试图添加第二个帖子时,我得到了一个例外, 对象引用未设置为对象的实例 ,关于 分类帖子 成员。

职位:

[Table(Name="tm_posts")]
public class Post : IDataErrorInfo
{
    public Post()
    {
        //Initialization of NOT NULL fields with their default values
    }

    [Column(Name = "id", DbType = "int", CanBeNull = false, IsDbGenerated = true, IsPrimaryKey = true)]
    public int ID { get; set; }

    private EntitySet<CategoryPost> _categoryRef = new EntitySet<CategoryPost>();
    [Association(Name = "tm_rel_categories_posts_fk2", IsForeignKey = true, Storage = "_categoryRef", ThisKey = "ID", OtherKey = "PostID")]
    public EntitySet<CategoryPost> CategoryRef
    {
        get { return _categoryRef; }
        set { _categoryRef.Assign(value); }
    }
}

分类帖子

[Table(Name = "tm_rel_categories_posts")]
public class CategoryPost
{
    [Column(Name = "id", DbType = "int", CanBeNull = false, IsDbGenerated = true, IsPrimaryKey = true)]
    public int ID { get; set; }

    [Column(Name = "fk_post", DbType = "int", CanBeNull = false)]
    public int PostID { get; set; }

    [Column(Name = "fk_category", DbType = "int", CanBeNull = false)]
    public int CategoryID { get; set; }

    private EntityRef<Post> _post = new EntityRef<Post>();
    [Association(Name = "tm_rel_categories_posts_fk2", IsForeignKey = true, Storage = "_post", ThisKey = "PostID", OtherKey = "ID")]
    public Post Post
    {
        get { return _post.Entity; }
        set { _post.Entity = value; }
    }

    private EntityRef<Category> _category = new EntityRef<Category>();
    [Association(Name = "tm_rel_categories_posts_fk", IsForeignKey = true, Storage = "_category", ThisKey = "CategoryID", OtherKey = "ID")]
    public Category Category
    {
        get { return _category.Entity; }
        set { _category.Entity = value; }
    }
}

类别

[Table(Name="tm_categories")]
public class Category
{
    [Column(Name = "id", DbType = "int", CanBeNull = false, IsDbGenerated = true, IsPrimaryKey = true)]
    public int ID { get; set; }

    [Column(Name = "fk_parent", DbType = "int", CanBeNull = true)]
    public int ParentID { get; set; }

    private EntityRef<Category> _parent = new EntityRef<Category>();
    [Association(Name = "tm_posts_fk2", IsForeignKey = true, Storage = "_parent", ThisKey = "ParentID", OtherKey = "ID")]
    public Category Parent
    {
        get { return _parent.Entity; }
        set { _parent.Entity = value; }
    }

    [Column(Name = "name", DbType = "varchar(100)", CanBeNull = false)]
    public string Name { get; set; }
}

我做错了什么?如何使插入不属于任何类别的文章成为可能?如何插入没有文章的类别?