Question

得到我认为在EF中的一个奇怪的行为，我希望有人可以阐明。基本上，如果我使用外键检索项目，则不会检索外键项？这似乎有点短暂。我是否遗漏了一些显而易见的事情，或者是否有一种模式可以解决这个问题？

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Entity;

namespace EFTest
{
    class Program
    {
        static void Main(string[] args)
        {
            Database.SetInitializer(new MCInitializer());

            EFTestEM context = new EFTestEM();
            var foos = from m in context.Foo
                    select m;

            foreach (var foo in foos)
            {
                // foo.MyBar is null?! How do I populate it?
                Console.WriteLine("{0},{1}",foo.Desc,foo.MyBar.Whatever);
            }

        }

    }


    [Table("tbl_Bar")]
    public class Bar
    {
        [Key,DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int BarId { get; set; }
        public string Whatever { get; set; }
        public string Whenever { get; set; }
    }


    [Table("tbl_Foo")]
    public class Foo
    {
        [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int FooId { get; set; }
        public string Desc { get; set; }
        public int MyBarId { get; set; }
        [ForeignKey("MyBarId")]
        public Bar MyBar { get; set; }

    }

    public class MCInitializer : DropCreateDatabaseAlways<EFTestEM>
    {
        protected override void Seed(EFTestEM context)
        {
            List<Bar> bars = new List<Bar>
                             {
                                 new Bar(){Whatever = "Bar1"},
                                 new Bar(){Whatever = "Bar2"},
                                 new Bar(){Whatever = "Bar3"},
                             };


            List<Foo> foos = new List<Foo>
                             {
                                 new Foo() {Desc = "Foo1", MyBar = bars[0]},
                                 new Foo() {Desc = "Foo2", MyBar = bars[1]},
                                 new Foo() {Desc = "Foo3", MyBar = bars[2]}
                             };

            foreach (var bar in bars)
                context.Bar.Add(bar);
            foreach (var foo in foos)
                context.Foo.Add(foo);

            context.SaveChanges();

            base.Seed(context);

        }
    }
}

Answer 1

对于延迟加载，您需要将相关属性设置为虚拟。

E.g。

public virtual Bar MyBar { get; set; }

Answer 2

相关属性需要“急切加载”。或者“懒得加载”＃39; （见查理的答案）。

要急切加载，代码需要使用＆＃39; Include＆＃39;扩展名：

        var foos = from m in context.Foo.Include("MyBar")
                select m;

EF代码优先 - 使用外键选择

2 个答案: