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:
- [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(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):
- Response.Write("-------------查询分类为1的版块-------------<br/>");
- 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 + "<br/>");
- Response.Write("-------------查询版块大于2个的分类-------------<br/>");
- 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 + "<br/>");
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