为Entity Framework导航属性设置默认对象 [英] Setting a default object for Entity Framework navigation property

问题描述

可以为Entity设置默认对象吗？

说我有一个 Person 原来没有要求个人资料。

现在我需要一个个人资料 - 但现有的实体当前没有个人资料。

有什么方法可以为将来加载的这些实体提供一个默认对象，所以任何使用 Person 实体可以假设配置文件永远不会为null，即使是默认值也是如此。

下面你可以看到我已经尝试了 - 它创建了一个默认值 - 但是即使数据库中有东西总是返回默认对象。

• 如果 Profile 是 null 我想返回一个默认的初始化对象


• 如果个人资料 不是 null 我想返回来自数据库的对象

另外 - 将默认对象附加到我的dbcontext的最明智的方法是什么？ p>

如何实现这个所需的行为？

  public class Person 
 {
 [Key] 
 public int Id {get; set;} 
 
 private Profile _profile; 
 public virtual Profile Profile 
 {
 get 
 {
 return _profile ?? （_profile = new Profile 
 {
 Person = this，
 PersonId = Id 
}）; 
} 
 set 
 {
 _profile = value; 
} 
 
 //属性
} 
} 
 
 public class Profile 
 {
 [Key，ForeignKey （Person）] 
 public int PersonId {get; set;} 
 
 [ForeignKey（PersonId）] 
 public virtual Person Person {get;组; } 
 
 //属性
} 
  

我知道你可以初始化集合，使其不为空，但我也想初始化一个对象。

解决方案

使用< a href =https://msdn.microsoft.com/en-us/library/system.data.objects.objectcontext.objectmaterialized(v=vs.110).aspx =nofollow> ObjectContext.ObjectMaterialized Event 。

在实现后，每个正在加载到上下文中的实体引发此事件。

在您的上下文的构造函数中，订阅此事件。在事件处理程序中，检查实体类型是否为 Person ，如果是，请为该人员创建新的配置文件。这是一个代码示例：

  public class Context：DbContext 
 {
 private readonly ObjectContext m_ObjectContext; 
 
 public DbSet< Person>人{get;组; } 
 public DbSet< Profile>个人资料{get;组; } 
 
 public Context（）
 {
 var m_ObjectContext =（（IObjectContextAdapter）this）.ObjectContext; 
 
 m_ObjectContext.ObjectMaterialized + = context_ObjectMaterialized; 
 
} 
 
 void context_ObjectMaterialized（object sender，System.Data.Entity.Core.Objects.ObjectMaterializedEventArgs e）
 {
 
 var人= e.Entity为人; 
 
 if（person == null）
 return; 
 
 if（person.Profile == null）
 person.Profile = new Profile（）{Person = person}; 
 
} 
} 
  

请注意，如果您提交更改，新配置文件将被保存回数据库。

Is it possible to set a default object for an Entity?

Say I have a Person entity that originally did not have a requirement for a Profile.

Now I require a Profile - but there are existing entities that do not currently have a Profile.

Is there a way I can provide a default object for these Entities when they're loaded in future, so anyone using the Person Entity can assume that Profile is never null and always has a value - even if it's a default.

Below you can see what I've tried - which does create a default value - but even when there is something in the database it always return the default object.

• If the Profile is null I want to return a default initialzied object
• If the Profile is not null I want to return the object from the database

Additionally - what would be the most sensible way to attach the "default" object to my dbcontext?

How can I achieve this desired behavior?

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

    private Profile _profile;
    public virtual Profile Profile
    {
        get
        {
            return _profile ?? (_profile= new Profile
            {
                Person = this,
                PersonId = Id
            });
        }
        set
        {
            _profile = value;
        }

        // properties
    }
}

public class Profile
{
    [Key, ForeignKey("Person")]
    public int PersonId {get; set;}

    [ForeignKey("PersonId")]
    public virtual Person Person{ get; set; }

    // properties
}

I know you can initialize collections so that they're not null, but I'd like to initialize a single object too.

解决方案

Use the ObjectContext.ObjectMaterialized Event.

This event is raised for each entity that is being loaded into context after materialization.

In the constructor for your context, subscribe to this event. And in the event handler, check if the entity type is Person, and if so, create the new profile for the person. Here is a code sample:

public class Context : DbContext
{
    private readonly ObjectContext m_ObjectContext;

    public DbSet<Person> People { get; set; }
    public DbSet<Profile> Profiles { get; set; }

    public Context()
    {
        var m_ObjectContext = ((IObjectContextAdapter)this).ObjectContext;

        m_ObjectContext.ObjectMaterialized += context_ObjectMaterialized;

    }

    void context_ObjectMaterialized(object sender, System.Data.Entity.Core.Objects.ObjectMaterializedEventArgs e)
    {

        var person = e.Entity as Person;

        if (person == null)
            return;

        if (person.Profile == null)
            person.Profile = new Profile() {Person = person};

    }
}

Please note that if you commit changes, the new profiles will be saved back to the database.

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