实体与实体框架之间的复杂关系 [英] Complex relationship between entities with Entity Framework

问题描述

也许是重复的，但我找不到这样的话题。

Maybe it is a duplicate but I couldn't find any topic like this.

我正在使用Entity Framework，并且数据库中有两个表：

I am using Entity Framework and have two tables in my database:

public class A
{
    public virtual B B1 { get; set; }
    public virtual B B2 { get; set; }
}

public class B
{
    public virtual A A1 { get; set; }
}

B1和A1或B2和A1之间没有关系。就像3种单向关系。您如何在Entity Framework中做到这一点？

And there is no relation between B1 and A1 or B2 and A1. It is like 3 one way relation. How can you do that in Entity Framework?

我遇到此错误：

在保存不为其关系公开外键属性的实体时发生错误

An error occurred while saving entities that do not expose foreign key properties for their relationships

有人知道如何处理吗？

预先感谢

推荐答案

因为您没有指出您正在使用的EF版本让我们看一下两个最新版本，即EF 6.2.0和EF-core 2.2.4。

Since you don't indicate in which EF version you're using let's look at the two current versions, EF 6.2.0 and EF-core 2.2.4.

有了EF6，这很容易。映射...

With EF6, it's easy. The mapping...

modelBuilder.Entity<A>().HasRequired(a => a.B1).WithMany();
modelBuilder.Entity<A>().HasRequired(a => a.B2).WithMany().WillCascadeOnDelete(false);
modelBuilder.Entity<B>().HasRequired(b => b.A1).WithMany().WillCascadeOnDelete(false);

...产生以下数据库模型（忽略索引）：

...produces the following database model (ignoring indexes):

CREATE TABLE [dbo].[A] (
    [ID] [int] NOT NULL IDENTITY,
    [B1_ID] [int] NOT NULL,
    [B2_ID] [int] NOT NULL,
    CONSTRAINT [PK_dbo.A] PRIMARY KEY ([ID])
)
CREATE TABLE [dbo].[B] (
    [ID] [int] NOT NULL IDENTITY,
    [A1_ID] [int] NOT NULL,
    CONSTRAINT [PK_dbo.B] PRIMARY KEY ([ID])
)

....其中 _ 是外键，其中之一可以级联删除。

....in which the fields with _ are foreign keys, one of which can have cascaded delete.

有了ef-core，它不是那么简单，甚至是越野车，似乎。第一个冲动是EF6等效项：

With ef-core, it's not so straightforward, even buggy, seemingly. The first impulse is the EF6 equivalent:

modelBuilder.Entity<A>().HasOne(a => a.B1).WithMany();
modelBuilder.Entity<A>().HasOne(a => a.B2).WithMany();
modelBuilder.Entity<B>().HasOne(b => b.A1).WithMany();

但是生成的模型不是人们期望的：

But the generated model isn't what one would expect:

  CREATE TABLE [B] (
      [ID] int NOT NULL IDENTITY,
      [A1ID] int NULL,
      CONSTRAINT [PK_B] PRIMARY KEY ([ID])
  );
  CREATE TABLE [A] (
      [ID] int NOT NULL,
      [B1ID] int NULL,
      CONSTRAINT [PK_A] PRIMARY KEY ([ID]),
      CONSTRAINT [FK_A_B_B1ID] FOREIGN KEY ([B1ID]) REFERENCES [B] ([ID]) ON DELETE NO ACTION,
      CONSTRAINT [FK_A_B_ID] FOREIGN KEY ([ID]) REFERENCES [B] ([ID]) ON DELETE CASCADE
  );
  ALTER TABLE [B] ADD CONSTRAINT [FK_B_A_A1ID] FOREIGN KEY ([A1ID]) REFERENCES [A] ([ID]) ON DELETE NO ACTION;

AB 关联之一被解释为1：1我认为这是一个错误。 WithMany 指令不应留下任何解释的空间。两个看似相同的映射会产生完全不同的数据库关系。

One of the A-B associations is interpreted as 1:1. In my opinion that's a bug. The WithMany instruction shouldn't leave any room for interpretation. Two seemingly identical mappings produce quite different database relationships. That can't be right.

这就是说，通过命名FK列很容易（但不是必须的）使EF处于正确的轨道上：

That said, it's easy (but shouldn't be necessary) to get EF on the right track by naming the FK columns:

modelBuilder.Entity<A>().HasOne(a => a.B1).WithMany().HasForeignKey("B1_ID")
    .IsRequired();
modelBuilder.Entity<A>().HasOne(a => a.B2).WithMany().HasForeignKey("B2_ID")
    .IsRequired().OnDelete(DeleteBehavior.Restrict);
modelBuilder.Entity<B>().HasOne(b => b.A1).WithMany().HasForeignKey("A1_ID")
    .IsRequired().OnDelete(DeleteBehavior.Restrict);

产生（忽略索引）：

  CREATE TABLE [B] (
      [ID] int NOT NULL IDENTITY,
      [A1_ID] int NOT NULL,
      CONSTRAINT [PK_B] PRIMARY KEY ([ID])
  );
  CREATE TABLE [A] (
      [ID] int NOT NULL IDENTITY,
      [B1_ID] int NOT NULL,
      [B2_ID] int NOT NULL,
      CONSTRAINT [PK_A] PRIMARY KEY ([ID]),
      CONSTRAINT [FK_A_B_B1_ID] FOREIGN KEY ([B1_ID]) REFERENCES [B] ([ID]) ON DELETE CASCADE,
      CONSTRAINT [FK_A_B_B2_ID] FOREIGN KEY ([B2_ID]) REFERENCES [B] ([ID]) ON DELETE NO ACTION
  );
  ALTER TABLE [B] ADD CONSTRAINT [FK_B_A_A1_ID] FOREIGN KEY ([A1_ID]) REFERENCES [A] ([ID]) ON DELETE NO ACTION;

请注意，必须显式设置必需的外键字段（如果有）。好吧，这只是实现细节。

Note that the foreign key fields must be set required explicitly (if they are). Well, that's just an implementation detail.

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