如何使用 EF6 正确设置多对多关系？

Posted 2023-04-12

【中文标题】如何使用 EF6 正确设置多对多关系？

【英文标题】：How to set up many to many relationship correctly with EF6?

【发布时间】：2020-01-24 14:30:05

【问题描述】：

我在让我的多对多关系发挥作用时遇到了一些问题。我有两个表，Beestje 和 Accessoire，我想显示它的内容。

这是我的桌子： 贝斯特杰：

public class Beestje

    public Beestje()
    
        this.Accessoires = new HashSet<Accessoire>();
    

    [Key]
    public int Id  get; set; 

    public String Naam  get; set; 

    public String Type  get; set; 

    public int Prijs  get; set; 

    public String Afbeelding  get; set; 

    public virtual ICollection<Accessoire> Accessoires  get; set; 

配饰：

public class Accessoire

    public Accessoire()
    
        this.Beestjes = new HashSet<Beestje>();
    

    [Key]
    public int Id  get; set; 

    public String Naam  get; set; 

    public double Prijs  get; set; 

    public String Afbeelding  get; set; 

    public virtual ICollection<Beestje> Beestjes  get; set; 

在我的控制器中，我从 Beestje 表中检索内容并将其返回给视图：

public ActionResult Beestjes()
    
        ViewBag.Message = "Your application beestjes page";

        List<Beestje> beestjes;

        using (var context = new MyContext())
        
            beestjes = context.Beestjes.ToList();

            return View(beestjes);
        
    

当我这样打印时没有问题：

@model IEnumerable<BeestjeOpJeFeestje.Models.Beestje>

<h2>@ViewBag.Message</h2>
<ul>
    @foreach (var a in Model)
    
        <li>@a.Naam</li>
    
</ul>

但是当我这样打印时出现错误：

@model IEnumerable<BeestjeOpJeFeestje.Models.Beestje>

<h2>@ViewBag.Message</h2>
<ul>
    @foreach (var a in Model)
    
        <li>@a.Naam</li>
        foreach (var b in a.Accessoires)
        
            <li>@b.Naam</li>
        
    
</ul>

在我调用 a.Accessoires 的部分出现问题。 错误是： "System.ObjectDisposedException: 'ObjectContext 实例已被释放，不能再用于需要连接的操作。'"

有人知道正确的方法，愿意提供正确的代码，或者给我一些指示吗？

提前致谢。

编辑：

我的上下文

public class MyContext : DbContext
    
        public MyContext() : base("name=Local")
        
            Database.SetInitializer(new MyContextInitializer());
        

        public DbSet<Beestje> Beestjes  get; set; 
        public DbSet<Accessoire> Accessoires  get; set; 

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        
            base.OnModelCreating(modelBuilder);
        
    

MyContextInitializer

public class MyContextInitializer : DropCreateDatabaseAlways<MyContext>
    
        protected override void Seed(MyContext context)
        
            base.Seed(context);

            context.Beestjes.Add(new Beestje()  Id = 1, Naam = "Aap", Type = "Jungle", Prijs = 150, Afbeelding = "Content/images/aap.png" );
            context.Beestjes.Add(new Beestje()  Id = 2, Naam = "Olifant", Type = "Jungle", Prijs = 250, Afbeelding = "Content/images/olifant.png" );
            context.Beestjes.Add(new Beestje()  Id = 3, Naam = "Zebra", Type = "Jungle", Prijs = 200, Afbeelding = "Content/images/zebra.png" );
            context.Beestjes.Add(new Beestje()  Id = 4, Naam = "Leeuw", Type = "Jungle", Prijs = 250, Afbeelding = "Content/images/leeuw.png" );
            context.Beestjes.Add(new Beestje()  Id = 5, Naam = "Hond", Type = "Boerderij", Prijs = 75, Afbeelding = "Content/images/doggo.png" );
            context.Beestjes.Add(new Beestje()  Id = 6, Naam = "Ezel", Type = "Boerderij", Prijs = 150, Afbeelding = "Content/images/donkey.png" );
            context.Beestjes.Add(new Beestje()  Id = 7, Naam = "Koe", Type = "Boerderij", Prijs = 100, Afbeelding = "Content/images/koe.png" );
            context.Beestjes.Add(new Beestje()  Id = 8, Naam = "Eend", Type = "Boerderij", Prijs = 50, Afbeelding = "Content/images/duck.png" );
            context.Beestjes.Add(new Beestje()  Id = 9, Naam = "Kuiken", Type = "Jungle", Prijs = 35, Afbeelding = "Content/images/kuiken.png" );
            context.Beestjes.Add(new Beestje()  Id = 10, Naam = "Pinguin", Type = "Sneeuw", Prijs = 175, Afbeelding = "Content/images/pingwing.png" );
            context.Beestjes.Add(new Beestje()  Id = 11, Naam = "IJsbeer", Type = "Sneeuw", Prijs = 300, Afbeelding = "Content/images/ijsbeer.png" );
            context.Beestjes.Add(new Beestje()  Id = 12, Naam = "Zeehond", Type = "Sneeuw", Prijs = 200, Afbeelding = "Content/images/zeehond.png" );
            context.Beestjes.Add(new Beestje()  Id = 13, Naam = "Kameel", Type = "Woestijn", Prijs = 175, Afbeelding = "Content/images/kameel.png" );
            context.Beestjes.Add(new Beestje()  Id = 14, Naam = "Slang", Type = "Sneeuw", Prijs = 125, Afbeelding = "Content/images/slang.png" );

            var accessoire1 = new Accessoire()  Naam = "Banaan", Prijs = 0.50 ;
            accessoire1.Beestjes.Add(context.Beestjes.Find(1));
            context.Accessoires.Add(accessoire1);

            var accessoire2 = new Accessoire()  Naam = "Zadel", Prijs = 4.50 ;
            accessoire2.Beestjes.Add(context.Beestjes.Find(3));
            context.Accessoires.Add(accessoire2);

            var accessoire3 = new Accessoire()  Naam = "Krukje", Prijs = 5 ;
            accessoire3.Beestjes.Add(context.Beestjes.Find(4));
            context.Accessoires.Add(accessoire3);

            var accessoire4 = new Accessoire()  Naam = "Zweep", Prijs = 2.50 ;
            accessoire4.Beestjes.Add(context.Beestjes.Find(4));
            context.Accessoires.Add(accessoire4);

            var accessoire5 = new Accessoire()  Naam = "Bal", Prijs = 2.50 ;
            accessoire5.Beestjes.Add(context.Beestjes.Find(5));
            accessoire5.Beestjes.Add(context.Beestjes.Find(12));
            context.Accessoires.Add(accessoire5);

            var accessoire6 = new Accessoire()  Naam = "Dansschoenen", Prijs = 3.00 ;
            accessoire6.Beestjes.Add(context.Beestjes.Find(10));
            context.Accessoires.Add(accessoire6);

            context.SaveChanges();
        
    

【问题讨论】：

ASP.NET MVC 是一个 Web 堆栈，而不是数据访问库。您使用的是 EF 还是 EF Core？哪个版本？请贴出MyContext的定义，就是配置关系的地方

是的，你是对的，我忘了包括那个。我正在使用 EF6。我现在在问题中包含了 MyContext。

至于错误本身，它没有说明关系。它说代码试图使用已经释放的 ObjectContext 实例。最新版本的 EF 使用 DbContext，而不是 ObjectContext。您是否在代码中的某处显式访问 ObjectContext？也许尝试使用 UDF？

我在我的项目中搜索了 ObjectContext，但我没有在任何地方使用它

【参考方案1】：

这很可能是由于对于每个Beestje，Accessoires 属性在您尝试在视图中访问它之前不会被加载，并且此时MyContext 已经被释放。

解决方案是使用.Include() 告诉 EF 立即加载它们，这就是所谓的“急切加载”：

using (var context = new MyContext())

    beestjes = context.Beestjes
        .Include("Accessoires")
        .ToList();

    return View(beestjes);

参考见https://docs.microsoft.com/en-us/ef/ef6/querying/related-data