Entity Framework 6 - 如何添加子实体？

Question

在 EF6 中添加子实体的正确方法是什么？为什么 Parent.Children.Add(child) 会抛出 NullReferenceException（Children 集合为 null）？如果 Parent.Children 集合至少有一项，则 .Add(child) 有效。我究竟做错了什么？这是我在 MVC 项目中的代码示例：

public class Parent
{
    public int ID { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Child> Children { get; set; }
}

public class Child
{
    public int ID { get; set; }
    public string Name { get; set; }
    public int ParentID { get; set; }
    public Parent Parent { get; set; }
}

public class AppContext : DbContext
{
    public AppContext() : base("AppContext") {}

    public DbSet<Parent> Parent { get; set; }
    public DbSet<Child> Children { get; set; }
}

public class AppContextInitializer : System.Data.Entity.DropCreateDatabaseAlways<AppContext>
{
    protected override void Seed(AppContext context)
    {
        Parent parent = new Parent { Name = "John" };
        Child child = new Child() { Name = "Mary" };
        parent.Children.Add(child); // Doesn't work: System.NullReferenceException, Children==null

        Parent parent2 = new Parent { Name = "Robert" };
        Child child2 = new Child { Name = "Sarah", Parent=parent2 };
        context.Children.Add(child2); // Works.... even inserts the parent entity thru the child entity!!

        context.SaveChanges();
    }
}

Answer 1

您需要在 Parent 的构造函数中初始化 Children 导航属性:

public class Parent
{
   public int ID { get; set; }
   public string Name { get; set; }
   public virtual ICollection<Child> Children { get; set; }

   public Parent()
   {
     Children=new List<Child>();
   }
}

Entity Framework 6 - 如何添加子实体？

Entity Framework 6 - How to Add Child Entities?

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