Linq to Sql 学习系列之八 继承与关系

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：

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]

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？