实体框架实体有一对多和一对一？ [英] Entity Framework Entity w/ One-to-Many and One-to-One?

问题描述

以下是一个父/子关系的简化示例，我试图在实体框架中首先工作。父母有一个孩子列表，其中一个被选为默认值：

The following is an simplified example of a parent/child relationship I'm trying to get working in Entity Framework code-first. The parent has a list of children, and one of them is selected as the default:

public class Parent
{
    [Key]
    public int Id { get; set; }
    public virtual Child DefaultChild { get; set; }          // One-to-One
    public virtual ICollection<Child> Children { get; set; } // One-to-Many
}

public class Child
{
    [Key]
    public int Id { get; set; }
    [Required]
    public virtual Parent Parent { get; set; }
}

当我运行实体框架的 add-migration 命令，它正在将Id for Child设置为一个键，而不是使其成为 IDENTITY ，它正在使用对父级的外键引用Id。

When I run entity framework's add-migration command, it is setting the Id for Child as a key, however, instead of making it an IDENTITY it is using a Foreign Key reference to the parent Id.

我尝试在DbContext上执行以下操作：

I tried doing the following on my DbContext:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder.Entity<Parent>().HasOptional(p => p.DefaultChild).WithRequired(c => c.Parent);
    modelBuilder.Entity<Parent>().HasMany(p => p.Children).WithRequired(c => c.Parent);
    base.OnModelCreating(modelBuilder);
} 

然而，它抛出了这个错误：指定的模式无效。错误：没有加载关系Sample.Presentation.DataAccess.Parent_DefaultChild，因为Sample.Presentation.DataAccess.Child类型不可用。

However, then it tosses this error: Schema specified is not valid. Errors: The relationship 'Sample.Presentation.DataAccess.Parent_DefaultChild' was not loaded because the type 'Sample.Presentation.DataAccess.Child' is not available.

你应该如何处理这些关系？

How are you supposed to handle these relationships?

我知道我可以为孩子添加一个布尔标志（例如 IsDefault ），并摆脱父对象的DefaultChild关系。但是，如果可能，我想避免这种情况。

I know I could add a boolean flag to the Child (e.g. IsDefault) and get rid of the DefaultChild relationship on the parent. However, I'd like to avoid that if possible.

编辑：我发现这个在StackOverflow上的信息，但是使用 WithOptionalPrincipal 它在孩子上创建一个额外的ParentId字段，而不是在父项上创建ChildId？不是我想要的。

I found this info on StackOverflow utilizing WithOptionalPrincipal, however, it creates an additional ParentId field on the child as opposed to creating the ChildId on the parent?! Not quite what I want.

编辑：添加 DatabaseGenerated as建议在评论。这就是迁移输出：

Added DatabaseGenerated as suggested in comments. This is what the migrations output:

CreateTable(
    "dbo.Children",
    c => new
       {
           Id = c.Int(nullable: false, identity: true),
           Parent_Id = c.Int(nullable: false),
       })
       .PrimaryKey(t => t.Id)
       .ForeignKey("dbo.Parents", t => t.Id)  // <== I don't want this!
       .ForeignKey("dbo.Parents", t => t.Parent_Id, cascadeDelete: true)
       .Index(t => t.Id)
       .Index(t => t.Parent_Id);

 CreateTable(
    "dbo.Parents",
    c => new
       {
           Id = c.Int(nullable: false, identity: true)  // <== I expect DefaultChildId after this
       })
       .PrimaryKey(t => t.Id);  // <== and a foreign key referencing Child.Id

推荐答案

一对一和一对多关系不能在两个实体上共存。因为EF要求一对一关系的从属关键（应该有外键）的关键字必须是这个规则确保一对一的关系工作。我认为这是一个可用的解决方案来添加一个属性IsDefault到子实体。

An one-to-one and an one-to-many relationship can not coexist on two entities.Because EF requires the key of the dependent end(should have foreign key) of an one-to-one relationship must be the foreign key to the principal end.This rule makes sure an one-to-one relationship work.I think it's an available solution to add a property,IsDefault,to the Child entity.

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