前言
为了演示继承与关系，我们创建一个论坛数据库，在数据库中创建三个表：
View Code
create DataBase ForumDataBase
go
--论坛版块分类表
create table Categories
(
CategoryID int identity(1,1) not null primary key,
CategoryName varchar(50) not null
)
select * from Categories
insert into Categories values('手机')
insert into Categories values('电脑')
insert into Categories values('生活')
--论坛版块表
create table Boards
(
BoardID int identity(1,1) not null primary key,
BoardName varchar(50) not null,
BoardCategory int not null ---对应论坛版块分类表的CategoryID
)
insert into Boards values('Nokia',1)
insert into Boards values('Iphone',1)
---论坛主题表
create table Topics
(
TopicID int identity(1,1) not null primary key,
TopicTitle varchar(50) not null,
TopicContent varchar(max) not null,
ParentTopic int null,
TopicType tinyint not null --0主题帖 1回复帖
)
drop table Topics
insert into Topics
values('Nokia 820首发','1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111',null,0)
insert into Topics values('111','22222222222222222222222222222222222222222222222222222222',1,1)
实体继承的定义
直接上代码：
namespace InheritConnection
{
[Table(Name="Topics")]
[InheritanceMapping(Code = 0, Type = typeof(NewTopic), IsDefault = true)]
[InheritanceMapping(Code = 1, Type = typeof(Reply))]
public class Topic
{
[Column(Name = "TopicID", DbType = "int identity", IsPrimaryKey = true, IsDbGenerated = true, CanBeNull = false)]
public int TopicID { set; get; }
[Column(Name = "TopicType", DbType = "tinyint", CanBeNull = false, IsDiscriminator = true)]
public int TopicType { set; get; }
[Column(Name = "TopicTitle", DbType = "varchar(50)", CanBeNull = false)]
public string TopicTitle { set; get; }
[Column(Name = "TopicContent", DbType = "varchar(max)", CanBeNull = false)]
public string TopicContent { set; get; }
}
public class NewTopic : Topic
{
public NewTopic()
{
base.TopicType = 0;
}
}
public class Reply : Topic
{
public Reply()
{
base.TopicType = 1;
}
[Column(Name = "ParentTopic", DbType = "int", CanBeNull = false)]
public int ParentTopic { get; set; }
}
}
1.首先定义的是Topic实体基类，然后两个子类的继承，NewTopic--主题帖，Reply--回复帖。
2.Topic类上的特性，下面先来看一下特性类
View Code
// 摘要:
// 映射 LINQ to SQL 应用程序中的继承层次结构。
[AttributeUsage(AttributeTargets.Class, AllowMultiple = true, Inherited = false)]
public sealed class InheritanceMappingAttribute : Attribute
{
// 摘要:
// 初始化 System.Data.Linq.Mapping.InheritanceMappingAttribute 类的一个新实例。
[TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
public InheritanceMappingAttribute();
// 摘要:
// 获取或设置映射的继承层次结构中的鉴别器代码值。
//
// 返回结果:
// 必须由用户指定。没有默认值。
public object Code { get; set; }
//
// 摘要:
// 获取或设置一个值，该值指示当鉴别器值与指定值不匹配时是否实例化此类型的对象。
//
// 返回结果:
// Default = false.
public bool IsDefault { get; set; }
//
// 摘要:
// 获取或设置层次结构中类的类型。
//
// 返回结果:
// 必须由用户指定。没有默认值。
public Type Type { get; set; }
}
通过type指定子类。
实体继承的使用
定义好继承的实体之后，我们就可以使用了。先是自定义一个DataContext吧：
namespace InheritConnection
{
public class BBSContext:DataContext
{
public Table<BoardCategory> BoardCategories;
public Table<Board> Boards;
public Table<Topic> Topics;
public BBSContext(string connection) : base(connection) { }
}
}
然后，我们来测试一下Linq to sql是否能根据TopicType识别派生类.
1.先来定义一个获取数据库链接配置的字符串常量
public static string ConnString = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
2.看一下在配置文件中定义的数据库链接字符串
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<connectionStrings>
<add name="ConnectionString" connectionString="Data Source=MWVCMNFFK0ZAZT5;Initial Catalog=ForumDataBase;Persist Security Info=True;User ID=sa;Password=saa;Pooling=False"
providerName="System.Data.SqlClient" />
</connectionStrings>
</configuration>
3.下面就通过如下代码进行测试一下
static void Main(string[] args)
{
BBSContext ctx = new BBSContext(ConnString);
var query = from t in ctx.Topics select t;
foreach (Topic topic in query)
{
if (topic is NewTopic)
{
NewTopic newtopic = topic as NewTopic;
Console.WriteLine("标题：" + newtopic.TopicTitle + " 类型：" + newtopic.TopicType);
}
else if (topic is Reply)
{
Reply reply = topic as Reply;
Console.WriteLine("标题：" + reply.TopicTitle + " 类型：" + reply.TopicType + " 隶属主题：" + reply.ParentTopic);
}
}
Console.WriteLine("SQL");
Console.ReadLine();
}
这是预先插入好的数据
现在可以F5运行查看结果
结果很给力，与我们查询分析器插入的数据保持一致。
再来看看如何进行增删操作：
NewTopic nt = new NewTopic() { TopicTitle = "还是新主题", TopicContent = "还是新主题"};
Reply rpl = new Reply() { TopicTitle = "还是新回复", TopicContent = "还是新回复", ParentTopic = 1 };
ctx.Topics.InsertOnSubmit(nt); ///添加
ctx.Topics.InsertOnSubmit(rpl);
ctx.SubmitChanges();
rpl = ctx.Topics.OfType<Reply>().Single(reply => reply.TopicID == 3);
ctx.Topics.DeleteOnSubmit(rpl); //删除
ctx.SubmitChanges();
var query1 = from t in ctx.Topics select t;
foreach (Topic topic in query1)
{
if (topic is NewTopic)
{
NewTopic newtopic = topic as NewTopic;
Console.WriteLine("标题：" + newtopic.TopicTitle + " 类型：" + newtopic.TopicType);
}
else if (topic is Reply)
{
Reply reply = topic as Reply;
Console.WriteLine("标题：" + reply.TopicTitle + " 类型：" + reply.TopicType + " 隶属主题：" + reply.ParentTopic);
}
}添加：先插入一条主题帖子，然后又插入一条回复帖。
删除：通过TopicID插入一条记录，然后删除已经之前的记录。
然后再次通过查询查看数据结果。
实体关系的定义
比如我们的论坛分类表和论坛版块表之间就有关系，这种关系是1对多的关系。也就是说一个论坛分类可能有多个论坛版块，这是很常见的。定义实体关系的优势在于，我们无须显式作连接操作就能处理关系表的条件。
首先来看看分类表的定义：
namespace InheritConnection
{
[Table(Name = "Categories")]
public class BoardCategory
{
[Column(Name = "CategoryID", DbType = "int identity", IsPrimaryKey = true, IsDbGenerated = true, CanBeNull = false)]
public int CategoryID { get; set; }
[Column(Name = "CategoryName", DbType = "varchar(50)", CanBeNull = false)]
public string CategoryName { get; set; }
private EntitySet<Board> _Boards;
[Association(OtherKey = "BoardCategory", Storage = "_Boards")]
public EntitySet<Board> Boards
{
get { return this._Boards; }
set { this._Boards.Assign(value); }
}
public BoardCategory()
{
this._Boards = new EntitySet<Board>();
}
}
}
CategoryID和CategoryName的映射没有什么不同，只是我们还增加了一个Boards属性，它返回的是Board实体集。通过特性，我们定义了关系外键为BoardCategory（Board表的一个字段）。然后来看看1对多，多端版块表的实体：
namespace InheritConnection
{
[Table(Name = "Boards")]
public class Board
{
[Column(Name = "BoardID", DbType = "int identity", IsPrimaryKey = true, IsDbGenerated = true, CanBeNull = false)]
public int BoardID { get; set; }
[Column(Name = "BoardName", DbType = "varchar(50)", CanBeNull = false)]
public string BoardName { get; set; }
[Column(Name = "BoardCategory", DbType = "int", CanBeNull = false)]
public int BoardCategory { get; set; }
private EntityRef<BoardCategory> _Category;
[Association(ThisKey = "BoardCategory", Storage = "_Category")]
public BoardCategory Category
{
get { return this._Category.Entity; }
set
{
this._Category.Entity = value;
value.Boards.Add(this);
}
}
}
}
在这里我们需要关联分类，设置了Category属性使用BoardCategory字段和分类表关联。
实体关系的使用
好了，现在我们就可以在查询句法中直接关联表了（数据库中不一定要设置表的外键关系）：
/////查看分类
var queryBoard = from b in ctx.Boards where b.Category.CategoryID == 1 select b;
foreach (Board b in queryBoard)
Console.WriteLine(b.BoardID + " " + b.BoardName);
Console.WriteLine("查看版块为两个以上的分类（包含两个）");
var query2 = from c in ctx.BoardCategories where c.Boards.Count >= 2 select c;
foreach (BoardCategory c in query2)
Console.WriteLine(c.CategoryID + " " + c.CategoryName + " " + c.Boards.Count);
Console.ReadLine();
下面是数据库中的测试数据
这是F5执行后的结果
可以看出上面并没有使用什么两个表之间的连接。
在添加分类的时候，如果这个分类下还有新的版块，那么提交新增分类的时候版块也会新增：
////添加分类
BoardCategory dbcat = new BoardCategory() { CategoryName = "三星" };
Board oracle = new Board() { BoardName = "小三", Category = dbcat };
ctx.BoardCategories.InsertOnSubmit(dbcat);
ctx.SubmitChanges();
