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論壇表結(jié)構(gòu)

為了演示繼承與關(guān)系，我們創(chuàng)建一個(gè)論壇數(shù)據(jù)庫(kù)，在數(shù)據(jù)庫(kù)中創(chuàng)建三個(gè)表：

1、論壇版塊分類表 dbo.Categories：

字段名	字段類型	可空	備注
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	備注
BoardID	int	not null	identity/主鍵
BoardName	varchar(50)	not null
BoardCategory	int	not null	對(duì)應(yīng)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	備注
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(gè)字段為null，否則就是所屬主題id
TopicType	tinyint	not null	0 – 主題貼 1 – 回復(fù)帖

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實(shí)體繼承的定義

Linq to sql支持實(shí)體的單表繼承，也就是基類和派生類都存儲(chǔ)在一個(gè)表中。對(duì)于論壇來(lái)說(shuō)，帖子有兩種，一種是主題貼，一種是回復(fù)帖。那么，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(shí)體的定義大家應(yīng)該很熟悉了。下面，我們?cè)賮?lái)定義兩個(gè)實(shí)體繼承帖子基類，分別是主題貼和回復(fù)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(duì)于主題貼，在數(shù)據(jù)庫(kù)中的TopicType就保存為0，而對(duì)于回復(fù)貼就保存為1。回復(fù)貼還有一個(gè)相關(guān)字段就是回復(fù)所屬主題貼的TopicID。那么，我們?cè)趺锤嬷?/span>Linq to sql在TopicType為0的時(shí)候識(shí)別為NewTopic，而1則識(shí)別為Reply那？只需稍微修改一下前面的Topic實(shí)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; }

}

為類加了InheritanceMapping特性定義，0的時(shí)候類型就是NewTopic，1的時(shí)候就是Reply。并且為TopicType字段上的特性中加了IsDiscriminator = true，告知Linq to sql這個(gè)字段就是用于分類的字段。

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實(shí)體繼承的使用

定義好繼承的實(shí)體之后，我們就可以使用了。先是自定義一個(gè)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，我們來(lái)測(cè)試一下Linq to sql是否能根據(jù)TopicType識(shí)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(biāo)題：" + newtopic.TopicTitle + " 類型：" + newtopic.TopicType + " ");

}

else if (topic is Reply)

{

Reply reply = topic as Reply;

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

}

然后我們往Topics表中加一些數(shù)據(jù)，如下圖：

啟動(dòng)程序得到如下測(cè)試結(jié)果：

當(dāng)然，你也可以在查詢句法中直接查詢派生實(shí)體：

newtopic和replies是兩個(gè)GridView控件，執(zhí)行效果如下圖：

再來(lái)看看如何進(jìn)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 = "還是新回復(fù)", TopicContent = "還是新回復(fù)", 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();

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