8,继承与关系

概要：

       对于一张表，可能存在多重类型。我们可以用继承来定义它们。

       实体关系的应用，解决数据表和表之间一些复杂问题。

内容：

      论坛主题表 dbo.Topics：

字段名	字段类型	可空	备注
TopicID	int	not null	identity/主键
TopicTitle	varchar(50)	not null
TopicContent	varchar(max)	not null
ParentTopic	int	null	如果帖子是主题贴这个字段为null，否则就是所属主题id
TopicType	tinyint	not null	0 – 主题贴 1 – 回复帖

定义实体类父类：包含都存在的部分字段。

                       然后定义可能出现的子类。

                       再定义各个类和字段的特性。

                       用[InheritanceMapping(。。)]判断类型

                       用[Column(IsDiscriminator=true)]标示判断依据

       //基类

   [Table(Name="Topics")]

   [InheritanceMapping(Code=0,Type=typeof(NewTopic),IsDefault=true)]

   [InheritanceMapping(Code=1,Type=typeof(Reply))]

   public class Topic

    {

       [Column(IsPrimaryKey=true,IsDbGenerated=true)]

       public int TopicID { get; set; }

       [Column]

       public string TopicTitle { get; set; }

       [Column]

       public string TopicContent { get; set; }

       [Column(IsDiscriminator=true)]

       public int TopicType { get; set; }

 

    }

   //新帖，TopicType=0,且主题字段ParentTopic为null,即不存在

   public class NewTopic : Topic

    {

       public NewTopic()

       {

           base.TopicType = 0;

       }

    }

   //回复贴，帖子类型TopicType=1,且主题字段为主题帖ID

   public class Reply : Topic

    {

       public Reply()

       {

           base.TopicType = 1;

       }

       [Column]

       public int ParentTopic { get; set; }

}

定义完实体类，下面就是使用方法，首先定义DataContext：

           //定义DataContext类

   public partial class BBSContext : DataContext

    {

       public Table<Topic> Topics;

       public BBSContext(string connection) : base(connection) { }

}

读出来Topic类，然后foreach逐行判断类型，然后再转换类型：

               BBSContext ctx = newBBSContext("");

           var query = from c in ctx.Topics select c;

           foreach(Topic t in query )

            {

                if (t is NewTopic)

                {

                    NewTopic newtopic = t asNewTopic;

                    //newtopic就是可以使用的NewTopic类型

                }

                else if(t is Reply)

                {

                   Reply reply = t asReply;

                    //reply就是可以使用的Reply类型

                }

           }

或直接使用：

       IEnumerable<NewTopic> tb = (from t inctx.Topics.OfType<NewTopic>() select t).ToList();

 

       Replyrpl =ctx.Topics.OfType<Reply>().Single(reply=> reply.TopicID == 8);

实体关系的使用：

      1、  论坛版块分类表 dbo.Categories：

字段名	字段类型	可空	备注
CategoryID	int	not null	identity/主键
CategoryName	varchar(50)	not null

2、  论坛版块表 dbo.Boards：

字段名	字段类型	可空	备注
BoardID	int	not null	identity/主键
BoardName	varchar(50)	not null
BoardCategory	int	not null	对应论坛版块分类表的CategoryID

实体类：

       [Table]

   public class Categories

    {

       public Categories()

        {

            this._Boards = newEntitySet<Boards>();

        }

        private EntitySet<Boards>_Boards;

        [Association(ThisKey="Category",Storage="_Boards")]

        public EntitySet<Boards> Boards

        {

            get { return this._Boards; }

            set { this._Boards.Assign(value); }

        }

 

       [Column(IsPrimaryKey=true,IsDbGenerated=true)]

       public int CategoryID { get; set; }

       [Column]

       public string CategoryName { get; set; }

    }

   [Table]

   public class Boards

    {

       [Association(ThisKey="Category",Storage="_Category")]

        public Categories Category

        {

            get { return this._Category.Entity;}

            set

            {

                this._Category.Entity = value;

                value.Boards.Add(this );

            }

        }

        private EntityRef<Categories>_Category;

 

        [Column(IsPrimaryKey=true,IsDbGenerated=true)]

       public int BoardID { get; set; }

       [Column]

       public string BoardName { get; set; }

       [Column]

       public int BoardCategory { get; set; }

}

使用：

              Response.Write("-------------查询分类为1的版块-------------<br/>");

       var query1 = fromb in ctx.Boards whereb.Category.CategoryID == 1 select b;

       foreach (Boardb in query1)

           Response.Write(b.BoardID + " " + b.BoardName + "<br/>");

       Response.Write("-------------查询版块大于2个的分类-------------<br/>");

       var query2 = fromc in ctx.BoardCategories where c.Boards.Count > 2 selectc;

       foreach (BoardCategoryc in query2)

           Response.Write(c.CategoryID + " " + c.CategoryName + " " +c.Boards.Count + "<br/>");

使用：

       DataLoadOptionsoptions = new DataLoadOptions();

       options.LoadWith<BoardCategory>(c => c.Boards);

        ctx.LoadOptions = options;

       Response.Write("-------------查询版块大于2个的分类-------------<br/>");

        varquery2 = from c inctx.BoardCategories where c.Boards.Count > 2select c;

        foreach(BoardCategory c inquery2)

           Response.Write(c.CategoryID + " " + c.CategoryName + " " +c.Boards.Count + "<br/>");

使用：

       BoardCategory dbcat= new BoardCategory(){ CategoryName = "Database" };

        Boardoracle = new Board(){ BoardName = "Oracle", Category =dbcat};

        ctx.BoardCategories.Add(dbcat);

       ctx.SubmitChanges();

 

　　这里下载源码