概要：
对于一张表，可能存在多重类型。我们可以用继承来定义它们。
实体关系的应用，解决数据表和表之间一些复杂问题。
内容：
论坛主题表 dbo.Topics：
|
字段名 |
字段类型 |
可空 |
备注 |
|
TopicID |
int |
not null |
identity/主键 |
|
TopicTitle |
varchar(50) |
not null |
|
|
TopicContent |
varchar(max) |
not null |
|
|
ParentTopic |
int |
null |
如果帖子是主题贴这个字段为null，否则就是所属主题id |
|
TopicType |
tinyint |
not null |
0 – 主题贴 1 – 回复帖 |
定义实体类父类：包含都存在的部分字段。
然后定义可能出现的子类。
再定义各个类和字段的特性。
用[InheritanceMapping(。。)]判断类型
用[Column(IsDiscriminator=true)]标示判断依据
//基类
[Table(Name="Topics")]
[InheritanceMapping(Code=0,Type=typeof(NewTopic),IsDefault=true)]
[InheritanceMapping(Code=1,Type=typeof(Reply))]
public class Topic
{
[Column(IsPrimaryKey=true,IsDbGenerated=true)]
public int TopicID { get; set; }
[Column]
public string TopicTitle { get; set; }
[Column]
public string TopicContent { get; set; }
[Column(IsDiscriminator=true)]
public int TopicType { get; set; }
}
//新帖，TopicType=0,且主题字段ParentTopic为null,即不存在
public class NewTopic : Topic
{
public NewTopic()
{
base.TopicType = 0;
}
}
//回复贴，帖子类型TopicType=1,且主题字段为主题帖ID
public class Reply : Topic
{
public Reply()
{
base.TopicType = 1;
}
[Column]
public int ParentTopic { get; set; }
}
定义完实体类，下面就是使用方法，首先定义DataContext：
//定义DataContext类
public partial class BBSContext : DataContext
{
public Table<Topic> Topics;
public BBSContext(string connection) : base(connection) { }
}
读出来Topic类，然后foreach逐行判断类型，然后再转换类型：
BBSContext ctx = newBBSContext("");
var query = from c in ctx.Topics select c;
foreach(Topic t in query )
{
if (t is NewTopic)
{
NewTopic newtopic = t asNewTopic;
//newtopic就是可以使用的NewTopic类型
}
else if(t is Reply)
{
Reply reply = t asReply;
//reply就是可以使用的Reply类型
}
}
或直接使用：
IEnumerable<NewTopic> tb = (from t inctx.Topics.OfType<NewTopic>() select t).ToList();
Replyrpl =ctx.Topics.OfType<Reply>().Single(reply=> reply.TopicID == 8);
实体关系的使用：
1、 论坛版块分类表 dbo.Categories：
|
字段名 |
字段类型 |
可空 |
备注 |
|
CategoryID |
int |
not null |
identity/主键 |
|
CategoryName |
varchar(50) |
not null |
|
2、 论坛版块表 dbo.Boards：
|
字段名 |
字段类型 |
可空 |
备注 |
|
BoardID |
int |
not null |
identity/主键 |
|
BoardName |
varchar(50) |
not null |
|
|
BoardCategory |
int |
not null |
对应论坛版块分类表的CategoryID |
实体类：
[Table]
public class Categories
{
public Categories()
{
this._Boards = newEntitySet<Boards>();
}
private EntitySet<Boards>_Boards;
[Association(ThisKey="Category",Storage="_Boards")]
public EntitySet<Boards> Boards
{
get { return this._Boards; }
set { this._Boards.Assign(value); }
}
[Column(IsPrimaryKey=true,IsDbGenerated=true)]
public int CategoryID { get; set; }
[Column]
public string CategoryName { get; set; }
}
[Table]
public class Boards
{
[Association(ThisKey="Category",Storage="_Category")]
public Categories Category
{
get { return this._Category.Entity;}
set
{
this._Category.Entity = value;
value.Boards.Add(this );
}
}
private EntityRef<Categories>_Category;
[Column(IsPrimaryKey=true,IsDbGenerated=true)]
public int BoardID { get; set; }
[Column]
public string BoardName { get; set; }
[Column]
public int BoardCategory { get; set; }
}
使用：
Response.Write("-------------查询分类为1的版块-------------<br/>");
var query1 = fromb in ctx.Boards whereb.Category.CategoryID == 1 select b;
foreach (Boardb 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 = fromc in ctx.BoardCategories where c.Boards.Count > 2 selectc;
foreach (BoardCategoryc in query2)
Response.Write(c.CategoryID + " " + c.CategoryName + " " +c.Boards.Count + "<br/>");
使用：
DataLoadOptionsoptions = new DataLoadOptions();
options.LoadWith<BoardCategory>(c => c.Boards);
ctx.LoadOptions = options;
Response.Write("-------------查询版块大于2个的分类-------------<br/>");
varquery2 = from c inctx.BoardCategories where c.Boards.Count > 2select c;
foreach(BoardCategory c inquery2)
Response.Write(c.CategoryID + " " + c.CategoryName + " " +c.Boards.Count + "<br/>");
使用：
BoardCategory dbcat= new BoardCategory(){ CategoryName = "Database" };
Boardoracle = new Board(){ BoardName = "Oracle", Category =dbcat};
ctx.BoardCategories.Add(dbcat);
ctx.SubmitChanges();