一步一步学linq to sql(八)继承与关系

古城微笑少年丶 2022-01-07 17:31 215阅读 0赞

**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：

2、 论坛版块表 dbo.Boards：

3、 论坛主题表 dbo.Topics：

<table> 
 <tbody> 
 <tr> 
 <td> 字段名 </td> 
 <td> 字段类型 </td> 
 <td> 可空 </td> 
 <td> 备注 </td> 
 </tr> 
 <tr> 
 <td> TopicID </td> 
 <td> int </td> 
 <td> not null </td> 
 <td> identity/主键 </td> 
 </tr> 
 <tr> 
 <td> TopicTitle </td> 
 <td> varchar(50) </td> 
 <td> not null </td> 
 <td> </td> 
 </tr> 
 <tr> 
 <td> TopicContent </td> 
 <td> varchar(max) </td> 
 <td> not null </td> 
 <td> </td> 
 </tr> 
 <tr> 
 <td> ParentTopic </td> 
 <td> int </td> 
 <td> null </td> 
 <td> 如果帖子是主题贴这个字段为null，否则就是所属主题id </td> 
 </tr> 
 <tr> 
 <td> TopicType </td> 
 <td> tinyint </td> 
 <td> not null </td> 
 <td> 0 – 主题贴 1 – 回复帖 </td> 
 </tr> 
 </tbody> 
</table>

**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> 
 <tbody> 
 <tr> 
 <td> [Table(Name = "Topics")] public class Topic { &nbsp;&nbsp;&nbsp; [Column(Name = "TopicID", DbType = "int identity", IsPrimaryKey = true, IsDbGenerated = true, CanBeNull = false)] &nbsp;&nbsp;&nbsp; public int TopicID { get; set; } &nbsp; &nbsp;&nbsp;&nbsp; [Column(Name = "TopicType", DbType = "tinyint", CanBeNull = false)] &nbsp;&nbsp;&nbsp; public int TopicType { get; set; } &nbsp; &nbsp;&nbsp;&nbsp; [Column(Name = "TopicTitle", DbType = "varchar(50)", CanBeNull = false)] &nbsp;&nbsp;&nbsp; public string TopicTitle { get; set; } &nbsp; &nbsp;&nbsp;&nbsp; [Column(Name = "TopicContent", DbType = "varchar(max)", CanBeNull = false)] &nbsp;&nbsp;&nbsp; public string TopicContent { get; set; } } </td> 
 </tr> 
 </tbody> 
</table>

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：

<table> 
 <tbody> 
 <tr> 
 <td> public class NewTopic : Topic { &nbsp;&nbsp;&nbsp; public NewTopic() &nbsp;&nbsp;&nbsp; { &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; base.TopicType = 0; &nbsp;&nbsp;&nbsp; } } &nbsp; public class Reply : Topic { &nbsp;&nbsp;&nbsp; public Reply() &nbsp;&nbsp;&nbsp; { &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; base.TopicType = 1; &nbsp;&nbsp;&nbsp; } &nbsp; &nbsp;&nbsp;&nbsp; [Column(Name = "ParentTopic", DbType = "int", CanBeNull = false)] &nbsp;&nbsp;&nbsp; public int ParentTopic { get; set; } } </td> 
 </tr> 
 </tbody> 
</table>

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> 
 <tbody> 
 <tr> 
 <td> [Table(Name = "Topics")] [InheritanceMapping(Code = 0, Type = typeof(NewTopic), IsDefault = true)] [InheritanceMapping(Code = 1, Type = typeof(Reply))] public class Topic { &nbsp;&nbsp;&nbsp; [Column(Name = "TopicID", DbType = "int identity", IsPrimaryKey = true, IsDbGenerated = true, CanBeNull = false)] &nbsp;&nbsp;&nbsp; public int TopicID { get; set; } &nbsp; &nbsp;&nbsp;&nbsp; [Column(Name = "TopicType", DbType = "tinyint", CanBeNull = false, IsDiscriminator = true)] &nbsp;&nbsp;&nbsp; public int TopicType { get; set; } &nbsp; &nbsp;&nbsp;&nbsp; [Column(Name = "TopicTitle", DbType = "varchar(50)", CanBeNull = false)] &nbsp;&nbsp;&nbsp; public string TopicTitle { get; set; } &nbsp; &nbsp;&nbsp;&nbsp; [Column(Name = "TopicContent", DbType = "varchar(max)", CanBeNull = false)] &nbsp;&nbsp;&nbsp; public string TopicContent { get; set; } } </td> 
 </tr> 
 </tbody> 
</table>

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：

<table> 
 <tbody> 
 <tr> 
 <td> public partial class BBSContext : DataContext { &nbsp;&nbsp;&nbsp; public Table&lt;BoardCategory&gt; BoardCategories; &nbsp;&nbsp;&nbsp; public Table&lt;Board&gt; Boards; &nbsp;&nbsp;&nbsp; public Table&lt;Topic&gt; Topics; &nbsp;&nbsp;&nbsp; public BBSContext(string connection) : base(connection) { } } </td> 
 </tr> 
 </tbody> 
</table>

然后，我们来测试一下Linq to sql是否能根据TopicType识别派生类：

<table> 
 <tbody> 
 <tr> 
 <td> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; BBSContext ctx = new BBSContext("server=xxx;database=BBS;uid=xxx;pwd=xxx"); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; var query = from t in ctx.Topics select t; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; foreach (Topic topic in query) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; { &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; if (topic is NewTopic) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; { &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; NewTopic newtopic = topic as NewTopic; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Response.Write("标题：" + newtopic.TopicTitle + " 类型：" + newtopic.TopicType + "&lt;br/&gt;"); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; } &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; else if (topic is Reply) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; { &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Reply reply = topic as Reply; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Response.Write("标题：" + reply.TopicTitle + " 类型：" + reply.TopicType + " 隶属主题：" + reply.ParentTopic + "&lt;br/&gt;"); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; } &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; } </td> 
 </tr> 
 </tbody> 
</table>

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：

<table> 
 <tbody> 
 <tr> 
 <td> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; IEnumerable newtopiclist = (from t in ctx.Topics.OfType&lt;NewTopic&gt;() select t).ToList(); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; newtopics.DataSource = newtopiclist; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; IEnumerable replylist = (from t in ctx.Topics.OfType&lt;Reply&gt;() select t).ToList(); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; replies.DataSource = replylist; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Page.DataBind(); </td> 
 </tr> 
 </tbody> 
</table>

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：

<table> 
 <tbody> 
 <tr> 
 <td> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; NewTopic nt = new NewTopic() { TopicTitle = "还是新主题", TopicContent = "还是新主题" }; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Reply rpl = new Reply() { TopicTitle = "还是新回复", TopicContent = "还是新回复", ParentTopic = 4 }; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ctx.Topics.Add(nt); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ctx.Topics.Add(rpl); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ctx.SubmitChanges(); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; rpl = ctx.Topics.OfType&lt;Reply&gt;().Single(reply =&gt; reply.TopicID == 8); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ctx.Topics.Remove(rpl); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ctx.SubmitChanges(); </td> 
 </tr> 
 </tbody> 
</table>

**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> 
 <tbody> 
 <tr> 
 <td> &nbsp; [Table(Name = "Categories")] public class BoardCategory { &nbsp;&nbsp;&nbsp; [Column(Name = "CategoryID", DbType = "int identity", IsPrimaryKey = true, IsDbGenerated = true, CanBeNull = false)] &nbsp;&nbsp;&nbsp; public int CategoryID { get; set; } &nbsp; &nbsp;&nbsp;&nbsp; [Column(Name = "CategoryName", DbType = "varchar(50)", CanBeNull = false)] &nbsp;&nbsp;&nbsp; public string CategoryName { get; set; } &nbsp; &nbsp;&nbsp;&nbsp; private EntitySet&lt;Board&gt; _Boards; &nbsp; &nbsp;&nbsp;&nbsp; [Association(OtherKey = "BoardCategory", Storage = "_Boards")] &nbsp;&nbsp;&nbsp; public EntitySet&lt;Board&gt; Boards &nbsp;&nbsp;&nbsp; { &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; get { return this._Boards; } &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; set { this._Boards.Assign(value); } &nbsp;&nbsp;&nbsp; } &nbsp; &nbsp;&nbsp;&nbsp; public BoardCategory() &nbsp;&nbsp;&nbsp; { &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; this._Boards = new EntitySet&lt;Board&gt;(); &nbsp;&nbsp;&nbsp; } } </td> 
 </tr> 
 </tbody> 
</table>

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> 
 <tbody> 
 <tr> 
 <td> &nbsp; [Table(Name = "Boards")] public class Board { &nbsp;&nbsp;&nbsp; [Column(Name = "BoardID", DbType = "int identity", IsPrimaryKey = true, IsDbGenerated = true, CanBeNull = false)] &nbsp;&nbsp;&nbsp; public int BoardID { get; set; } &nbsp; &nbsp;&nbsp;&nbsp; [Column(Name = "BoardName", DbType = "varchar(50)", CanBeNull = false)] &nbsp;&nbsp;&nbsp; public string BoardName { get; set; } &nbsp; &nbsp;&nbsp;&nbsp; [Column(Name = "BoardCategory", DbType = "int", CanBeNull = false)] &nbsp;&nbsp;&nbsp; public int BoardCategory { get; set; } &nbsp; &nbsp;&nbsp;&nbsp; private EntityRef&lt;BoardCategory&gt; _Category; &nbsp; &nbsp;&nbsp;&nbsp; [Association(ThisKey = "BoardCategory", Storage = "_Category")] &nbsp;&nbsp;&nbsp; public BoardCategory Category &nbsp;&nbsp;&nbsp; { &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; get { return this._Category.Entity; } &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; set &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; { &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; this._Category.Entity = value; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; value.Boards.Add(this); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; } } } </td> 
 </tr> 
 </tbody> 
</table>

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）：

<table> 
 <tbody> 
 <tr> 
 <td> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Response.Write("-------------查询分类为1的版块-------------&lt;br/&gt;"); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; var query1 = from b in ctx.Boards where b.Category.CategoryID == 1 select b; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; foreach (Board b in query1) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Response.Write(b.BoardID + " " + b.BoardName + "&lt;br/&gt;"); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Response.Write("-------------查询版块大于2个的分类-------------&lt;br/&gt;"); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; var query2 = from c in ctx.BoardCategories where c.Boards.Count &gt; 2 select c; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; foreach (BoardCategory c in query2) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Response.Write(c.CategoryID + " " + c.CategoryName + " " + c.Boards.Count + "&lt;br/&gt;"); </td> 
 </tr> 
 </tbody> 
</table>

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：

<table> 
 <tbody> 
 <tr> 
 <td> 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] &nbsp; SELECT [t0].[CategoryID], [t0].[CategoryName] FROM [Categories] AS [t0] WHERE (( &nbsp;&nbsp;&nbsp; SELECT COUNT(*) &nbsp;&nbsp;&nbsp; FROM [Boards] AS [t1] &nbsp;&nbsp;&nbsp; WHERE [t1].[BoardCategory] = [t0].[CategoryID] &nbsp;&nbsp;&nbsp; )) &gt; @p0 -- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [2] &nbsp; 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] </td> 
 </tr> 
 </tbody> 
</table>

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：

<table> 
 <tbody> 
 <tr> 
 <td> DataLoadOptions options = new DataLoadOptions(); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; options.LoadWith&lt;BoardCategory&gt;(c =&gt; c.Boards); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ctx.LoadOptions = options; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Response.Write("-------------查询版块大于2个的分类-------------&lt;br/&gt;"); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; var query2 = from c in ctx.BoardCategories where c.Boards.Count &gt; 2 select c; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; foreach (BoardCategory c in query2) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Response.Write(c.CategoryID + " " + c.CategoryName + " " + c.Boards.Count + "&lt;br/&gt;"); </td> 
 </tr> 
 </tbody> 
</table>

查询经过改造后会得到下面的SQL：

<table> 
 <tbody> 
 <tr> 
 <td> SELECT [t0].[CategoryID], [t0].[CategoryName], [t1].[BoardID], [t1].[BoardName], [t1].[BoardCategory], ( &nbsp;&nbsp;&nbsp; SELECT COUNT(*) &nbsp;&nbsp;&nbsp; FROM [Boards] AS [t3] &nbsp;&nbsp;&nbsp; WHERE [t3].[BoardCategory] = [t0].[CategoryID] &nbsp;&nbsp;&nbsp; ) AS [count] FROM [Categories] AS [t0] LEFT OUTER JOIN [Boards] AS [t1] ON [t1].[BoardCategory] = [t0].[CategoryID] WHERE (( &nbsp;&nbsp;&nbsp; SELECT COUNT(*) &nbsp;&nbsp;&nbsp; FROM [Boards] AS [t2] &nbsp;&nbsp;&nbsp; WHERE [t2].[BoardCategory] = [t0].[CategoryID] &nbsp;&nbsp;&nbsp; )) &gt; @p0 ORDER BY [t0].[CategoryID], [t1].[BoardID] -- @p0: Input Int32 (Size = 0; Prec = 0; Scale = 0) [2] </td> 
 </tr> 
 </tbody> 
</table>

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：

<table> 
 <tbody> 
 <tr> 
 <td> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; BoardCategory dbcat = new BoardCategory() { CategoryName = "Database" }; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Board oracle = new Board() { BoardName = "Oracle", Category = dbcat}; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ctx.BoardCategories.Add(dbcat); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ctx.SubmitChanges(); </td> 
 </tr> 
 </tbody> 
</table>

上述代码导致下面的SQL被执行：

<table> 
 <tbody> 
 <tr> 
 <td> INSERT INTO [Categories]([CategoryName]) VALUES (@p0) &nbsp; SELECT [t0].[CategoryID] FROM [Categories] AS [t0] WHERE [t0].[CategoryID] = (SCOPE_IDENTITY()) &nbsp; -- @p0: Input AnsiString (Size = 8; Prec = 0; Scale = 0) [Database] &nbsp; INSERT INTO [Boards]([BoardName], [BoardCategory]) VALUES (@p0, @p1) &nbsp; SELECT [t0].[BoardID] FROM [Boards] AS [t0] WHERE [t0].[BoardID] = (SCOPE_IDENTITY()) &nbsp; -- @p0: Input AnsiString (Size = 6; Prec = 0; Scale = 0) [Oracle] -- @p1: Input Int32 (Size = 0; Prec = 0; Scale = 0) [23] </td> 
 </tr> 
 </tbody> 
</table>

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？

转载于:https://www.cnblogs.com/kingdom\_0/articles/2050140.html