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：
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[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>();
}
}
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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：
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[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);
}
}
}
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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。