一步一步学Linq to sql（八）：继承与关系
[2] 一步一步学Linq to sql（八）：继承与关系
[3] 一步一步学Linq to sql（八）：继承与关系
[4] 一步一步学Linq to sql（八）：继承与关系
[5] 一步一步学Linq to sql（八）：继承与关系
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一步一步学Linq to sql（二）：DataContext与实体
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：
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[Table(Name = "Topics")] public class Topic { [Column(Name = "TopicID", DbType = "int identity", IsPrimaryKey = true, IsDbGenerated = true, CanBeNull = false)] public int TopicID { get; set; }
[Column(Name = "TopicType", DbType = "tinyint", CanBeNull = false)] public int TopicType { get; set; }
[Column(Name = "TopicTitle", DbType = "varchar(50)", CanBeNull = false)] public string TopicTitle { get; set; }
[Column(Name = "TopicContent", DbType = "varchar(max)", CanBeNull = false)] public string TopicContent { get; set; } } |
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：
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public class NewTopic : Topic { public NewTopic() { base.TopicType = 0; } }
public class Reply : Topic { public Reply() { base.TopicType = 1; }
[Column(Name = "ParentTopic", DbType = "int", CanBeNull = false)] public int ParentTopic { get; set; } } |
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：
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[Table(Name = "Topics")] [InheritanceMapping(Code = 0, Type = typeof(NewTopic), IsDefault = true)] [InheritanceMapping(Code = 1, Type = typeof(Reply))] public class Topic { [Column(Name = "TopicID", DbType = "int identity", IsPrimaryKey = true, IsDbGenerated = true, CanBeNull = false)] public int TopicID { get; set; }
[Column(Name = "TopicType", DbType = "tinyint", CanBeNull = false, IsDiscriminator = true)] public int TopicType { get; set; }
[Column(Name = "TopicTitle", DbType = "varchar(50)", CanBeNull = false)] public string TopicTitle { get; set; }
[Column(Name = "TopicContent", DbType = "varchar(max)", CanBeNull = false)] public string TopicContent { get; set; } } |
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。
