Linq实体关系简单概述

本文向大家介绍Linq实体关系，可能好多人还不了解Linq实体关系，没有关系，看完本文你肯定有不少收获，希望本文能教会你更多东西。

Linq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Linq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：

1. [Table(Name = "Categories")]
2. public class BoardCategory
3. {
4. [Column(Name = "CategoryID", DbType = "int identity",
   IsPrimaryKey = true, IsDbGenerated = true, CanBeNull = false)]
5. public int CategoryID { get; set; }
6. [Column(Name = "CategoryName", DbType = "varchar(50)", CanBeNull = false)]
7. public string CategoryName { get; set; }
8. private EntitySet<Board> _Boards;
9. [Association(OtherKey = "BoardCategory", Storage = "_Boards")]
10. public EntitySet<Board> Boards
11. {
12. get { return this._Boards; }
13. set { this._Boards.Assign(value); }
14. }
15. public BoardCategory()
16. {
17. this._Boards = new EntitySet<Board>();
18. }
19. }

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：

1. [Table(Name = "Boards")]
2. public class Board
3. {
4. [Column(Name = "BoardID", DbType = "int identity", IsPrimaryKey = true,
   IsDbGenerated = true, CanBeNull = false)]
5. public int BoardID { get; set; }
6. [Column(Name = "BoardName", DbType = "varchar(50)", CanBeNull = false)]
7. public string BoardName { get; set; }
8. [Column(Name = "BoardCategory", DbType = "int", CanBeNull = false)]
9. public int BoardCategory { get; set; }
10. private EntityRef<BoardCategory> _Category;
11. [Association(ThisKey = "BoardCategory", Storage = "_Category")]
12. public BoardCategory Category
13. {
14. get { return this._Category.Entity; }
15. set
16. {
17. this._Category.Entity = value;
18. value.Boards.Add(this);
19. }
20. }
21. }

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。

Linq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）：

1. 1. Response.Write("-------------查询分类为1的版块-------------<br/>");
   2. var query1 = from b in ctx.Boards where b.Category.CategoryID == 1 select b;
   3. foreach (Board b in query1)
   4. Response.Write(b.BoardID + " " + b.BoardName + "<br/>");
   5. Response.Write("-------------查询版块大于2个的分类-------------<br/>");
   6. var query2 = from c in ctx.BoardCategories where c.Boards.Count > 2 select c;
   7. foreach (BoardCategory c in query2)
   8. Response.Write(c.CategoryID + " " + c.CategoryName + " " + c.Boards.Count + "<br/>");

转载于:https://www.cnblogs.com/Peter-Luo/archive/2012/05/31/2528947.html