Castle ActiveRecord（三） 关联映射

ActiveRecord 提供一个DomainModel 工具，它使我们创建对象模型的相互关系非常容易。在上面的例子中我们已经创建了一个Blog类，它完全可以提供一个Post类及它们之间的关联。平白点讲，他们的关联是：

• 一个Post属于一个Blog
• 一个Blog有多个Post

使用HasMany特性 和BelongsTo特性

必须使用HasMany和BelongTo特性表达对象之间的关联。如下：

[ActiveRecord( " Blogs " )]
public   class  Blog : ActiveRecordBase
{

    private IList _posts;

    [HasMany(typeof(Post), Table="posts", ColumnKey="post_blogid")]
    public IList Posts
    {
        get { return _posts; }
        set { _posts = value; }
    }
}

[ActiveRecord( " Posts " )]
public   class  Post : ActiveRecordBase
{

    private Blog _blog;

    [BelongsTo("post_blogid")]
    public Blog Blog
    {
        get { return _blog; }
        set { _blog = value; }
    }
}

在这个例子中，由于Post有一个BelongTo联合，ActiveRecord能够收集到这个信息并创建正确的HasMany，因此不必指定所有信息，象下面这样就可以了，注意对比“[HasMany(typeof(Post))]”：

[ActiveRecord( " Blogs " )]
public   class  Blog : ActiveRecordBase
{

    private IList _posts;

    [HasMany(typeof(Post))]
    public IList Posts
    {
        get { return _posts; }
        set { _posts = value; }
    }
}

不要感到疑惑，让我们看看Post类，注意[BelongsTo("post_blogid")]：

using  System;
using  System.Collections.Generic;
using  System.Text;

namespace  ActiveRecord
{
   using Castle.ActiveRecord;
    [ActiveRecord("Posts")]
    public class Post : ActiveRecordBase
    {

        private int _postId;
        private string _postTitle;
        private string _postContents;
        private string _postCategory;
        private System.DateTime _postCreated;
        private bool _postPublished;
        private Blog _blog;

        [PrimaryKey(PrimaryKeyType.Native, "post_id")]
        public int PostId
        {
            get
            {
                return this._postId;
            }
            set
            {
                this._postId = value;
            }
        }

        [Property(Column = "post_title")]
        public string PostTitle
        {
            get
            {
                return this._postTitle;
            }
            set
            {
                this._postTitle = value;
            }
        }

        [Property(Column = "post_contents")]
        public string PostContents
        {
            get
            {
                return this._postContents;
            }
            set
            {
                this._postContents = value;
            }
        }

        [Property(Column = "post_category")]
        public string PostCategory
        {
            get
            {
                return this._postCategory;
            }
            set
            {
                this._postCategory = value;
            }
        }

        [Property(Column = "post_created")]
        public System.DateTime PostCreated
        {
            get
            {
                return this._postCreated;
            }
            set
            {
                this._postCreated = value;
            }
        }

        [Property(Column = "post_published")]
        public bool PostPublished
        {
            get
            {
                return this._postPublished;
            }
            set
            {
                this._postPublished = value;
            }
        }

        [BelongsTo("post_blogid")]
        public Blog Blog
        {
            get
            {
                return this._blog;
            }
            set
            {
                this._blog = value;
            }
        }

        public static void DeleteAll()
        {
            ActiveRecordBase.DeleteAll(typeof(Post));
        }

        public static Post[] FindAll()
        {
            return ((Post[])(ActiveRecordBase.FindAll(typeof(Post))));
        }

        public static Post Find(int PostId)
        {
            return ((Post)(ActiveRecordBase.FindByPrimaryKey(typeof(Post), PostId)));
        }
    }
}

好，我们来看看怎么使用关联映射吧。先看下面的代码：

Blog blog  =  Blog.Find( 1 );
newPost.PostTitle  =   " MyPostTitle " ;
newPost.PostPublished  =   true ;
newPost.PostContents  =   " This is a great post! " ;
newPost.PostCategory  =   " Blogging " ;
newPost.PostCreated  =  DateTime.Now;
blog.Posts.Add(newPost );
blog.Save(); 

这段代码是无法正常运行的，因为Post类是一个瞬态类，在使用时我们必须先持久化它，象下面这样就可以运行了：

Blog blog  =  Blog.Find( 1 );
Post newPost  =   new  Post();
newPost.PostTitle  =   " MyPostTitle " ;
newPost.PostPublished  =   true ;
newPost.PostContents  =   " This is a great post! " ;
newPost.PostCategory  =   " Blogging " ;
newPost.PostCreated  =  DateTime.Now;
newPost.Blog  =  blog;
newPost.Save();
blog.Posts.Add(newPost);
blog.Save(); 

不过你的Blogs表中要有记录才行，否则会发生异常，告诉你找不到。又有人不禁想出了下面的代码：

Blog blog  =  New Blog();
blog.BlogName = " blogname " ;
blog.BlogAuthor = " This is my first post " );
Post newPost  =   new  Post();
newPost.PostTitle  =   " MyPostTitle " ;
newPost.PostPublished  =   true ;
newPost.PostContents  =   " This is a great post! " ;
newPost.PostCategory  =   " Blogging " ;
newPost.PostCreated  =  DateTime.Now;
newPost.Save();
blog.Posts.Add(newPost);
blog.Save(); 

这样还是不行地，在向数据库里面保存时，Post实际需要的是Blog的Id，而此时Blog还没有Id。

上面的代码都使用了Blog端的关联，也可以象下面这样使用：

Blog blog  =  Blog.Find( 1 );
Post newPost  =   new  Post();
newPost.PostTitle  =   " MyPostTitle " ;
newPost.PostPublished  =   true ;
newPost.PostContents  =   " This is a great post! " ;
newPost.PostCategory  =   " Blogging " ;
newPost.PostCreated  =  DateTime.Now;
newPost.Blog  =  blog;
newPost.Blog  =  blog;  //  Linking them
newPost.Save();

处理blog的时候，Posts不会自动地得到通知。如果想应用最新的改变，也好就是说要想使用新实例化的post，必须“刷新”它。我们可以通过设置级联变化来使用它：

[HasMany( typeof (Post), Table = " Posts " , ColumnKey = " post_blogid " , Cascade = ManyRelationCascadeEnum.SaveUpdate)]
public  IList Posts
{
    get { return _posts; }
    set { _posts = value; }
}

这样的开头那段代码就能运行了：

Blog blog  =  Blog.Find( 1 );
newPost.PostTitle  =   " MyPostTitle " ;
newPost.PostPublished  =   true ;
newPost.PostContents  =   " This is a great post! " ;
newPost.PostCategory  =   " Blogging " ;
newPost.PostCreated  =  DateTime.Now;
blog.Posts.Add(newPost );
blog.Save(); 

怎么样，一定有点晕了吧，不要紧，下一节我们将讲解一下HasManyAttribute和BelongsToAttribute。