本系列課程均轉自

LoveCherry

技術無極限http://www.cnblogs.com/lovecherry/archive/2007/08/14/855681.html

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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：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？

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]

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