entity-framework – EF Linq to entities查询为TPC CTP5代码优先实体生成UNION

2023年2月20日 194次阅读

我有以下实体：

public class User
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class HappyUser : User
{
    public bool IsHappy { get; set; }
}

我正在使用Table Per Concrete Type(TPC)配置实体,以生成User表和HappyUser表.我希望HappyUser表包含User类的属性,我不希望这两个表之间有任何关系.

我按如下方式配置实体：

public class UserTest : DbContext
{
    public DbSet<User> Users { get; set; }
    public DbSet<HappyUser> HappyUsers { get; set; }


    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<HappyUser>().Map(m =>
        {
            m.MapInheritedProperties();
            m.ToTable("HappyUser");
        });
    }
}

这些表是正确生成的,但是当我查询表时,EF会在User和HappyUser表上生成UNION.查询如下：

        UserTest db = new UserTest();

        var users = from u in db.Users
                    select u;

        var happyUsers = from u in db.Users.OfType<HappyUser>()
                         select u;

用户的SQL生成UNION.这不是我期望或想要的.我想简单地从Users表中检索行.

SELECT 
CASE WHEN ([UnionAll1].[C2] = 1) THEN '0X' ELSE '0X0X' END AS [C1], 
[UnionAll1].[Id] AS [C2], 
[UnionAll1].[Name] AS [C3], 
CASE WHEN ([UnionAll1].[C2] = 1) THEN CAST(NULL AS bit) ELSE [UnionAll1].[C1] END AS [C4]
FROM  (SELECT 
 [Extent1].[Id] AS [Id], 
 [Extent1].[Name] AS [Name], 
 CAST(NULL AS bit) AS [C1], 
 cast(1 as bit) AS [C2]
 FROM [dbo].[User] AS [Extent1]
UNION ALL
 SELECT 
 [Extent2].[Id] AS [Id], 
 [Extent2].[Name] AS [Name], 
 [Extent2].[IsHappy] AS [IsHappy], 
 cast(0 as bit) AS [C1]
 FROM [dbo].[HappyUser] AS [Extent2]) AS [UnionAll1]

HappyUsers的SQL按预期工作.

SELECT 
'0X0X' AS [C1], 
[Extent1].[Id] AS [Id], 
[Extent1].[Name] AS [Name], 
[Extent1].[IsHappy] AS [IsHappy]
FROM [dbo].[HappyUser] AS [Extent1]

我有什么想法我做错了吗？或者这是EF的缺陷？

最佳答案 HappyUsers是用户.因此,db.Users应该返回两者. EF在这里是正确的.

但是,EF确实有一个限制：没有办法(在L2E中,无论如何)只返回一种类型的结果.由于Liskov Substitution Principal,您通常不想这样做.但如果你这样做,you can do it in ESQL, and there are (somewhat tedious) workarounds for L2E.

底线：如果您发现自己想要这样做,请重新考虑您的设计.在这种情况下,继承可能不是正确的关系.

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