Question

我得出结论，我应该在我的Code-First设计中定义独立协会和外键协会。例如：

public class Book
{
  public int ID {get; set;}
  public int AuthorID {get; set;}
  [ForeignKey("AuthorID")]
  public Author Author {get; set;} 
}

根据上面的定义，当我想要更改图书的作者时，是否必须更新AuthorID，或者仅使用以下行就足够了？
myBook.Author =作者;
如果这是我第一次为这本书定义作者，我会在上面的行中得到一个null异常吗？（当我为其分配一些值时，EF会自动初始化图书的作者吗？）我应该在定义中对其进行初始化：

代码：

public class Book
{
  public int ID {get; set;}
  public int AuthorID {get; set;}

  private Author m_Author;
  [ForeignKey("AuthorID")]
  public Author Author {get
  {
    get
    {
      if (m_Author == null)
        m_Author = new Author();
      return m_Author;
    }
    set
    {
      this.m_Author = value;
    }
  } 
}

Answer 1

首先，您不能同时使用independent and foreign key association - 您可以使用第一个或第二个。区别在于您是否使用FK属性。如果使用外键关联，则应使用外键来建立关系。这就是为什么在EFv4中引入FK关联的原因。

编辑：

使用自定义POCO（EFv4.1中常见）和FK关系时，为什么要使用FK而不是导航属性的简单示例：

这没有任何问题：

var child = new ChildEntity() {Id = 1};
child.ParentEntityId = 1;  // Assigning FK
context.Childs.Attach(child);
context.Entry(child).State = EntityState.Modified;
context.SaveChanges();

抛出异常：

var parent = new ParentEntity() { Id = 1 };
context.Parents.Attach(parent);
var child = new ChildEntity() {Id = 1};
child.Parent = parent;  // <-- Assigning only navigation property
// Next line will cause InvalidOperationException:
// A referential integrity constraint violation occurred: 
// The property values that define the referential constraints 
// are not consistent between principal and dependent objects in 
// the relationship.
context.Childs.Attach(child);
context.Entry(child).State = EntityState.Modified;
context.SaveChanges();

这也没有任何问题：

var parent = new ParentEntity() { Id = 1 };
context.Parents.Attach(parent);
var child = new ChildEntity() {Id = 1};
child.Parent = parent;
child.ParentEntityId = 1; // <-- AGAIN assigning FK
context.Childs.Attach(child);
context.Entry(child).State = EntityState.Modified;
context.SaveChanges();

Answer 2

以下示例具有相同的问题：

public class PingPongPlayer
{
    [Key]
    public string Name { get; set; }
    public string EMail { get; set; }
    public int Ranking { get; set; }
}

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

    public string FrkPlayer1 { get; set; }
    public string FrkPlayer2 { get; set; }

    [ForeignKey("FrkPlayer1")]
    public PingPongPlayer Player1 { get; set; }

    [ForeignKey("FrkPlayer2")]
    public PingPongPlayer Player2 { get; set; }

    public DateTime MatchDate { get; set; }

    public bool AlreadyPlayed { get; set; }

    public string Player1Name
    {
        get { return Player1.Name; }
    }

    public string Player2Name
    {
        get { return Player2.Name; }
    }
}

如果我将Control的属性绑定到Player1Name属性，我会得到NullPointerException。在数据库中，我可以看到表，它似乎有正确的键值。

Name    EMail   Ranking <br>
a   a@q.com 10 <br>
b   b@q.com 15 <br>
c   c@q.com 12 <br>
d   d@q.com 20 <br>

Id  FrkPlayer1  FrkPlayer2  MatchDate   AlreadyPlayed
1   a           b           2011-04-21 00:00:00.000 0
2   a           c           2011-04-21 00:00:00.000 0
3   b           c           2011-04-21 00:00:00.000 0
4   a           d           2011-04-21 00:00:00.000 0
5   a           c           2011-04-21 00:00:00.000 0
6   d           c           2011-04-21 00:00:00.000 0

要解决问题，请更换：

[ForeignKey("FrkPlayer1")]
public PingPongPlayer Player1 { get; set; }

[ForeignKey("FrkPlayer2")]
public PingPongPlayer Player2 { get; set; }

通过

[ForeignKey("FrkPlayer1")]
public virtual PingPongPlayer Player1 { get; set; }

[ForeignKey("FrkPlayer2")]
public virtual PingPongPlayer Player2 { get; set; }

更新实体框架4.1代码优先中的外键关联

2 个答案: