Entity Framework Core 3.0 - 创建自引用多对多关系

Question

所以这是我的问题......我试图创建的是一种自我引用的多对多关系。基本上这是我的模型。

public class InformationSystem
{
  public InformationSystem()
  {
     Systems = new HashSet<InformationSystem>();
     ParentSystems = new HashSet<InformationSystem>();
  }
 [Key()]
 public int InformationSystemID { get; set; }
 public string InformationSystemName { get; set; }
 //Navigation properties
 public virtual ICollection<InformationSystem> Systems { get; set; }
 public virtual ICollection<InformationSystem> ParentSystems { get; set; }
}

这个想法是，一个系统可以有许多父母，而一个父母可以有许多孩子。我知道如何创建一个自引用实体，其中许多孩子可以有一个父母。让我困惑的是多对多的部分。下面是我的 DbContext。

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
  modelBuilder.Entity<InformationSystem>(entity =>
  {
    entity
       .HasMany(e => e.ParentSystems)
       .WithMany(e => e.Systems)
       .OnDelete(DeleteBehavior.Restrict);
  });

然而，在我的 DbContext 上，我收到一个错误，即 .WithMany 不包含 with Many 的定义，该定义将接受类型集合的输入。我知道，当代码首先创建迁移并更新数据库时，基本上需要构建的是链接表。我认为链接表有两列，没有键。一列是 InformationSystemID，一列是 ParentInformationSystemID。两者都是外键。我还知道，要使其正常工作，应限制删除行为，以便在链接表中删除或更新条目时，该更改不会级联（并创建循环）。有人可以指出我需要做什么才能让 EF Core 3 正确执行此操作的正确方向吗？如果我必须自己创建一个链接表，我该怎么做？我需要在 DbContext 中做什么？我知道链接表看起来像这样：
我非常感激。

public class InfoSysToParentInfoSys
{
  public int InfoSysID;
  public virtual InformationSystem InfoSys;

  public int ParentInfoSysID;
  public virtual InformationSystem ParentInfoSys;
}

Answer 1

在 EF Core 中，需要在模型中包含一个实体来表示 M:N 关系中的联接表，然后将导航属性添加到指向联接实体的多对多关系的任一侧。

新表：

public class InformationSystem
{
    public InformationSystem()
    {
    }

    [Key()]
    public virtual int InformationSystemID { get; set; }
    public virtual string InformationSystemName { get; set; }

    public virtual ICollection<InformationSystemRelation> Systems { get; set; }
    public virtual ICollection<InformationSystemRelation> ParentSystems { get; set; }

}


public class InformationSystemRelation
{
    public int ParentId { get; set; }
    public InformationSystem Parent { get; set; }

    public int ChildId { get; set; }
    public InformationSystem Child { get; set; }
}

映射：

modelBuilder.Entity<InformationSystemRelation>()
    .HasKey(x => new { x.ParentId, x.ChildId });

modelBuilder.Entity<InformationSystemRelation>()
    .HasOne(x => x.Parent)
    .WithMany(x => x.Systems)
    .HasForeignKey(x => x.ParentId)                    
    .OnDelete(DeleteBehavior.Restrict);


modelBuilder.Entity<InformationSystemRelation>()
    .HasOne(x => x.Child)
    .WithMany(x => x.ParentSystems)
    .HasForeignKey(x => x.ChildId)
    .OnDelete(DeleteBehavior.Restrict);

整个样本：

class Program
{
    static void Main(string[] args)
    {
        var db = new MyDbContext();

        var is1 = new InformationSystem() { InformationSystemName = "is1" };
        var is2 = new InformationSystem() { InformationSystemName = "is2" };
        var is3 = new InformationSystem() { InformationSystemName = "is3" };
        var is4 = new InformationSystem() { InformationSystemName = "is4" };

        db.InformationSystems.Add(is1);
        db.InformationSystems.Add(is2);
        db.InformationSystems.Add(is3);
        db.InformationSystems.Add(is4);

        db.SaveChanges();

        var r1 = new InformationSystemRelation() { ParentId = 1, ChildId = 2 };
        var r2 = new InformationSystemRelation() { ParentId = 1, ChildId = 3 };
        var r3 = new InformationSystemRelation() { ParentId = 4, ChildId = 2 };
        var r4 = new InformationSystemRelation() { ParentId = 2, ChildId = 3 };
        var r5 = new InformationSystemRelation() { ParentId = 2, ChildId = 4 };

        db.InformationSystemRelations.Add(r1);
        db.InformationSystemRelations.Add(r2);
        db.InformationSystemRelations.Add(r3);
        db.InformationSystemRelations.Add(r4);
        db.InformationSystemRelations.Add(r5);

        db.SaveChanges();

        var o2 = db.InformationSystems.Include(x => x.Systems).Include(x => x.ParentSystems).Single(x => x.InformationSystemID == 2);
    }

}

public class InformationSystem
{
    public InformationSystem()
    {
    }

    [Key()]
    public virtual int InformationSystemID { get; set; }
    public virtual string InformationSystemName { get; set; }

    public virtual ICollection<InformationSystemRelation> Systems { get; set; }
    public virtual ICollection<InformationSystemRelation> ParentSystems { get; set; }

}

public class MyDbContext : DbContext
{

    public DbSet<InformationSystem> InformationSystems { get; set; }
    public DbSet<InformationSystemRelation> InformationSystemRelations { get; set; }


    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<InformationSystem>(entity =>
        {
            modelBuilder.Entity<InformationSystemRelation>()
                    .HasKey(x => new { x.ParentId, x.ChildId });

            modelBuilder.Entity<InformationSystemRelation>()
                .HasOne(x => x.Parent)
                .WithMany(x => x.Systems)
                .HasForeignKey(x => x.ParentId)
                .OnDelete(DeleteBehavior.Restrict);

            modelBuilder.Entity<InformationSystemRelation>()
                .HasOne(x => x.Child)
                .WithMany(x => x.ParentSystems)
                .HasForeignKey(x => x.ChildId)
                .OnDelete(DeleteBehavior.Restrict);
        });
    }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer("data source=(local)\\SQLEXPRESS;Initial catalog=Test;Integrated security=SSPI");
        base.OnConfiguring(optionsBuilder);
    }
}

public class InformationSystemRelation
{
    public int ParentId { get; set; }
    public InformationSystem Parent { get; set; }

    public int ChildId { get; set; }
    public InformationSystem Child { get; set; }
}