实体框架0..1至0关系 [英] Entity Framework 0..1 to 0 relation

问题描述

class First
{
    [Key]
    public int Id { get; set; }
}

class Second
{
    [Key]
    public int Id { get; set; }

    public int? First_Id { get; set; }

    [ForeignKey("First_Id")]
    public First First { get; set; }
}

public class SecondMapping : EntityTypeConfiguration<Second>
{
    public SecondMapping () 
        : base()
    {
        this.HasOptional(s => s.First)
            .With ... ???
    }
}

第二个可能有一个参考First。但是首先从来没有提到第二。是否可以使用Entity Framework 4.1应用此映射？

Second may have a reference to First. But First never has a reference to Second. Is it possible to apply this mapping with Entity Framework 4.1?

编辑：
以前，这是我的解决方案：

Previously, that was my solution:

this.HasOptional(s => s.First)
    .WithOptionalDependent()
    .WillCascadeOnDelete(false);

第二个可以包含一个First的实例（取决于某种使用属性）。首先不包含Second的任何实例。

Second could contain one instance of First (dependent on some kind of Usage-Attribute). First doesn't contain any instance of Second.

推荐答案

只有外键也是一个一对一的关系依赖实体的关键。所以正确的映射是：

One-to-one relation is possible only if foreign key is also primary key of dependent entity. So the correct mapping is:

class First
{
    [Key]
    public int Id { get; set; }
}

class Second
{
    [Key, ForeignKey("First")]
    public int Id { get; set; }
    public First First { get; set; }
}

原因是在数据库中强制执行一对一的关系外键在 Second 实体中必须是唯一的。但实体框架不支持唯一的密钥 - 唯一唯一的EF值是主键。这是EF的限制。

The reason is that to enforce one-to-one relation in the database foreign key must be unique in the Second entity. But entity framework doesn't support unique keys - the only unique value for EF is primary key. This is limitation of EF.

在 Morteza Manavi的博客。解决方法是基于映射关联作为一对多，并通过在自定义数据库初始化程序中引入唯一约束来强制实现数据库中的唯一性。

There is workaround described on Morteza Manavi's blog. Workaround is based on mapping association as one-to-many and enforcing uniqueness in database by introducing unique constraints in custom database initializer.

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