论坛表结构
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：
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
3、 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
实体继承的定义
Linq to sql支持实体的单表继承，也就是基类和派生类都存储在一个表中。对于论坛来说，帖子有两种，一种是主题贴，一种是回复帖。那么，我们就先定义帖子基类：
[Table(Name = "Topics")]
public class Topic
{
[Column(Name = "TopicID", DbType = "int identity", IsPrimaryKey = true, IsDbGenerated = true, CanBeNull = false)]
public int TopicID { get; set; }
[Column(Name = "TopicType", DbType = "tinyint", CanBeNull = false)]
public int TopicType { get; set; }
[Column(Name = "TopicTitle", DbType = "varchar(50)", CanBeNull = false)]
public string TopicTitle { get; set; }
[Column(Name = "TopicContent", DbType = "varchar(max)", CanBeNull = false)]
public string TopicContent { get; set; }
}
这些实体的定义大家应该很熟悉了。下面，我们再来定义两个实体继承帖子基类，分别是主题贴和回复贴：
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; }
}
对于主题贴，在数据库中的TopicType就保存为0，而对于回复贴就保存为1。回复贴还有一个相关字段就是回复所属主题贴的TopicID。那么，我们怎么告知Linq to sql在TopicType为0的时候识别为NewTopic，而1则识别为Reply那？只需稍微修改一下前面的Topic实体定义：
[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 { get; set; }
[Column(Name = "TopicType", DbType = "tinyint", CanBeNull = false, IsDiscriminator = true)]
public int TopicType { get; set; }
[Column(Name = "TopicTitle", DbType = "varchar(50)", CanBeNull = false)]
public string TopicTitle { get; set; }
[Column(Name = "TopicContent", DbType = "varchar(max)", CanBeNull = false)]
public string TopicContent { get; set; }
}
为类加了InheritanceMapping特性定义，0的时候类型就是NewTopic，1的时候就是Reply。并且为TopicType字段上的特性中加了IsDiscriminator = true，告知Linq to sql这个字段就是用于分类的字段。
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：
public partial 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：
BBSContext ctx = new BBSContext("server=xxx;database=BBS;uid=xxx;pwd=xxx");
var query = from t in ctx.Topics select t;
foreach (Topic topic in query)
{
if (topic is NewTopic)
{
NewTopic newtopic = topic as NewTopic;
Response.Write("标题：" + newtopic.TopicTitle + " 类型：" + newtopic.TopicType + "<br/>");
}
else if (topic is Reply)
{
Reply reply = topic as Reply;
Response.Write("标题：" + reply.TopicTitle + " 类型：" + reply.TopicType + " 隶属主题：" + reply.ParentTopic + "<br/>");
}
}
然后我们往Topics表中加一些数据，如下图：
启动程序得到如下测试结果：
当然，你也可以在查询句法中直接查询派生实体：
IEnumerable newtopiclist = (from t in ctx.Topics.OfType<NewTopic>() select t).ToList();
newtopics.DataSource = newtopiclist;
IEnumerable replylist = (from t in ctx.Topics.OfType<Reply>() select t).ToList();
replies.DataSource = replylist;
Page.DataBind();
newtopic和replies是两个GridView控件，执行效果如下图：
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 = 4 };
ctx.Topics.Add(nt);
ctx.Topics.Add(rpl);
ctx.SubmitChanges();
rpl = ctx.Topics.OfType<Reply>().Single(reply => reply.TopicID == 8);
ctx.Topics.Remove(rpl);
ctx.SubmitChanges();
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：
[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：
[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）：
Response.Write("-------------查询分类为1的版块-------------<br/>");
var query1 = from b in ctx.Boards where b.Category.CategoryID == 1 select b;
foreach (Board b 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 = from c in ctx.BoardCategories where c.Boards.Count > 2 select c;
foreach (BoardCategory c in query2)
Response.Write(c.CategoryID + " " + c.CategoryName + " " + c.Boards.Count + "<br/>");
在数据库中加一些测试数据，如下图：
运行程序后得到下图的结果：
我想定义实体关系的方便我不需要再用语言形容了吧。执行上述的程序会导致下面SQL的执行：
SELECT [t0].[BoardID], [t0].[BoardName], [t0].[BoardCategory]
FROM [Boards] AS [t0]
INNER JOIN [Categories] AS [t1] ON [t1].[CategoryID] = [t0].[BoardCategory]
WHERE [t1].[CategoryID] = @p0
-- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [1]
SELECT [t0].[CategoryID], [t0].[CategoryName]
FROM [Categories] AS [t0]
WHERE ((
SELECT COUNT(*)
FROM [Boards] AS [t1]
WHERE [t1].[BoardCategory] = [t0].[CategoryID]
)) > @p0
-- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [2]
SELECT [t0].[BoardID], [t0].[BoardName], [t0].[BoardCategory]
FROM [Boards] AS [t0]
WHERE [t0].[BoardCategory] = @p0
-- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [1]
可以看到，第二个查询并没有做外连接，还记得DataLoadOptions吗？我们可以要求Linq to sql在读取版块分类信息的时候也把版块信息一起加载：今天就讲到这里。大家可以自己尝试为帖子表也定义实体的关系，因为，是不是可以直接通过帖子获取帖子下的回复，或者直接通过回复得到所属帖子那？
DataLoadOptions options = 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/>");
var query2 = from c in ctx.BoardCategories where c.Boards.Count > 2 select c;
foreach (BoardCategory c in query2)
Response.Write(c.CategoryID + " " + c.CategoryName + " " + c.Boards.Count + "<br/>");
查询经过改造后会得到下面的SQL：
SELECT [t0].[CategoryID], [t0].[CategoryName], [t1].[BoardID], [t1].[BoardName], [t1].[BoardCategory], (
SELECT COUNT(*)
FROM [Boards] AS [t3]
WHERE [t3].[BoardCategory] = [t0].[CategoryID]
) AS [count]
FROM [Categories] AS [t0]
LEFT OUTER JOIN [Boards] AS [t1] ON [t1].[BoardCategory] = [t0].[CategoryID]
WHERE ((
SELECT COUNT(*)
FROM [Boards] AS [t2]
WHERE [t2].[BoardCategory] = [t0].[CategoryID]
)) > @p0
ORDER BY [t0].[CategoryID], [t1].[BoardID]
-- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [2]
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：
BoardCategory dbcat = new BoardCategory() { CategoryName = "Database" };
Board oracle = new Board() { BoardName = "Oracle", Category = dbcat};
ctx.BoardCategories.Add(dbcat);
ctx.SubmitChanges();
上述代码导致下面的SQL被执行：
INSERT INTO [Categories]([CategoryName]) VALUES (@p0)
SELECT [t0].[CategoryID]
FROM [Categories] AS [t0]
WHERE [t0].[CategoryID] = (SCOPE_IDENTITY())
-- @p0: Input AnsiString (Size = 8; Prec = 0; Scale = 0) [Database]
INSERT INTO [Boards]([BoardName], [BoardCategory]) VALUES (@p0, @p1)
SELECT [t0].[BoardID]
FROM [Boards] AS [t0]
WHERE [t0].[BoardID] = (SCOPE_IDENTITY())
-- @p0: Input AnsiString (Size = 6; Prec = 0; Scale = 0) [Oracle]
-- @p1: Input Int32 (Size = 0; Prec = 0; Scale = 0) [23]
