Linq to Sql 學習系列之八繼承與關係

論壇表結構

爲了演示繼承與關係，我們創建一個論壇數據庫，在數據庫中創建三個表：

1、論壇版塊分類表 dbo.Categories ：

字段名	字段類型	可空	備註
CategoryID	int	not null	identity/ 主鍵
CategoryName	varchar(50)	not null

2、論壇版塊表 dbo.Boards ：

字段名	字段類型	可空	備註
BoardID	int	not null	identity/ 主鍵
BoardName	varchar(50)	not null
BoardCategory	int	not null	對應論壇版塊分類表的 CategoryID

3、論壇主題表 dbo.Topics ：

字段名	字段類型	可空	備註
TopicID	int	not null	identity/ 主鍵
TopicTitle	varchar(50)	not null
TopicContent	varchar(max)	not null
ParentTopic	int	null	如果帖子是主題貼這個字段爲 null ，否則就是所屬主題 id
TopicType	tinyint	not null	0 – 主題貼 1 – 回覆帖

實體繼承的定義

Linq to sql 支持實體的單表繼承，也就是基類和派生類都存儲在一個表中。對於論壇來說，帖子有兩種，一種是主題貼，一種是回覆帖。那麼，我們就先定義帖子基類：

[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 ; }

}

這些實體的定義大家應該很熟悉了。下面，我們再來定義兩個實體繼承帖子基類，分別是主題貼和回覆貼：

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 ; }

}

對於主題貼，在數據庫中的 TopicType 就保存爲 0 ，而對於回覆貼就保存爲 1 。回覆貼還有一個相關字段就是回覆所屬主題貼的 TopicID 。那麼，我們怎麼告知 Linq to sql 在 TopicType 爲 0 的時候識別爲 NewTopic ，而 1 則識別爲 Reply 那？只需稍微修改一下前面的 Topic 實體定義：

[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 ; }

}

爲類加了 InheritanceMapping 特性定義， 0 的時候類型就是 NewTopic ， 1 的時候就是 Reply 。並且爲 TopicType 字段上的特性中加了 IsDiscriminator = true ，告知 Linq to sql 這個字段就是用於分類的字段。

實體繼承的使用

定義好繼承的實體之後，我們就可以使用了。先是自定義一個 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) { }

}

然後，我們來測試一下 Linq to sql 是否能根據 TopicType 識別派生類：

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(" 標題： " + newtopic.TopicTitle + " 類型： " + newtopic.TopicType + " " );

}

else if (topic is Reply )

{

Reply reply = topic as Reply ;

Response.Write(" 標題： " + reply.TopicTitle + " 類型： " + reply.TopicType + " 隸屬主題： " + reply.ParentTopic + " " );

}

然後我們往 Topics 表中加一些數據，如下圖：

啓動程序得到如下測試結果：

當然，你也可以在查詢句法中直接查詢派生實體：

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 是兩個 GridView 控件，執行效果如下圖：

再來看看如何進行增刪操作：

NewTopic nt = new NewTopic () { TopicTitle = " 還是新主題 " , TopicContent = " 還是新主題 " };

Reply rpl = new Reply () { TopicTitle = " 還是新回覆 " , TopicContent = " 還是新回覆 " , 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();

實體關係的定義

比如我們的論壇分類表和論壇版塊表之間就有關係，這種關係是 1 對多的關係。也就是說一個論壇分類可能有多個論壇版塊，這是很常見的。定義實體關係的優勢在於，我們無須顯式作連接操作就能處理關係表的條件。

首先來看看分類表的定義：

[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 的映射沒有什麼不同，只是我們還增加了一個 Boards 屬性，它返回的是 Board 實體集。通過特性，我們定義了關係外鍵爲 BoardCategory （ Board 表的一個字段）。然後來看看 1 對多，多端版塊表的實體：

[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 );

}

在這裏我們需要關聯分類，設置了 Category 屬性使用 BoardCategory 字段和分類表關聯。

實體關係的使用

好了，現在我們就可以在查詢句法中直接關聯表了（數據庫中不一定要設置表的外鍵關係）：

Response.Write("------------- 查詢分類爲 1 的版塊 ------------- " );

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 + " " );

Response.Write("------------- 查詢版塊大於 2 個的分類 ------------- " );

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 + " " );

在數據庫中加一些測試數據，如下圖：

運行程序後得到下圖的結果：

我想定義實體關係的方便我不需要再用語言形容了吧。執行上述的程序會導致下面 SQL 的執行：

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，第二個查詢並沒有做外連接，還記得 DataLoadOptions 嗎？我們可以要求 Linq to sql 在讀取版塊分類信息的時候也把版塊信息一起加載：

DataLoadOptions options = new DataLoadOptions ();

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

ctx.LoadOptions = options;

Response.Write("------------- 查詢版塊大於 2 個的分類 ------------- " );

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 + " " );

查詢經過改造後會得到下面的 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]

在添加分類的時候，如果這個分類下還有新的版塊，那麼提交新增分類的時候版塊也會新增：

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

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

ctx.BoardCategories.Add(dbcat);

ctx.SubmitChanges();

上述代碼導致下面的 SQL 被執行：

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]

今天就講到這裏。大家可以自己嘗試爲帖子表也定義實體的關係，因爲，是不是可以直接通過帖子獲取帖子下的回覆，或者直接通過回覆得到所屬帖子那？

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Linq to Sql 學習系列之八 繼承與關係

Linq to Sql 學習系列之八繼承與關係