实体框架关系未从数据库加载 [英] Entity Framework relation not loaded from database

问题描述

我正在使用Entity Framework 6 Code First，并且遇到了多对多关系未正确同步到我的内部对象的问题。

I'm playing around with Entity Framework 6 Code First and have the problem that a many-to-many relationship is not correctly synchronized to my "internal" objects.

我有以下课程：

public class ClassA
{
    public String AId { get; set; }
    public ICollection<ClassB> Bs { get; set; }
    public ICollection<ClassC> Cs { get; set; }
}
public class ClassB
{
    public int BId { get; set; }
    ICollection<ClassA> As { get; set; }
}
public class ClassC
{
    public int CId { get; set; }
    public ClassA A { get; set; }
}

A和B之间存在多对多关系，其中A和C有一对多的关系。我使用Fluent API构建映射：

A and B have a many-to-many relationship where A and C have a one-to-many relationship. I use Fluent API to build the mappings:

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

    modelBuilder.Entity<ClassA>().ToTable("ClassA");
    modelBuilder.Entity<ClassA>().HasKey(a => a.AId);
    modelBuilder.Entity<ClassA>().HasManyClassB>(a => a.Bs).WithMany(b => b.As).Map(
            a =>
            {
                a.MapLeftKey("AId");
                a.MapRightKey("BId");
                a.ToTable("ClassAClassB");
            });

    modelBuilder.Entity<ClassA>().Property(a => a.OrderId).IsVariableLength().HasColumnName("AId");
    modelBuilder.Entity<ClassB>().ToTable("ClassB");
    modelBuilder.Entity<ClassB>().HasKey(b => b.BId);
    modelBuilder.Entity<ClassB>().Property(b => b.BId)
         .HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity)
         .HasColumnName("BId");
    modelBuilder.Entity<ClassC>().ToTable("ClassC");
    modelBuilder.Entity<ClassC>().HasKey(c => c.CId);
    modelBuilder.Entity<ClassC>().HasRequired(c => c.A).WithMany(a => a.Bs).Map(m => m.MapKey("AId"));
    modelBuilder.Entity<ClassC>().Property(c => c.CId)    
         .HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity)
         .HasColumnName("CId");
}

如果我创建ClassA，ClassB和ClassC类的对象，请将它们添加到我的数据库上下文并调用SaveChanges（）。在我看来，值已正确存储在数据库中。

If i create objects of class ClassA, ClassB and ClassC, add them to my database context and invoke SaveChanges(). In my opinion the values are stored in the database correctly:

MariaDB [Test]> select * from ClassA;
+----------+
|   BId    |
+----------+
| 2017-002 |
+----------+

MariaDB [Test]> select * from ClassB;
+--------+
|   BId  |
+--------+
|    2   |
+--------+

MariaDB [OMSData]> select * from ClassAClassB;
+----------+-----+
|    AId   | BId |
+----------+-----+
| 2017-002 |  2  |
+----------+-----+

MariaDB [Test]> select * from classC;
+----+-----------+
| CId |    AId   |
+-----+----------+
|  1  | 2017-002 |
+-----+----------+

但是，如果我再次执行测试程序，关系将无法正确恢复。

However, if I execute my test program another time, the relationships are not correctly restored.

这意味着：

• ClassA中的B集合不包含任何对象（反之亦然）。


• 我只能通过ClassA访问ClassC的对象

我是否必须手动调用同步，或者模型定义中有错误？

Do I have to invoke the synchronization manually or is there an error in model definition?

预先感谢！

推荐答案

您的课程不是正确的。从逻辑上考虑，您需要将多对多绑定存储在某个地方，这意味着您需要存储多对多关系的第三个实体类。

You are not building your classes right. Think logically, you need to store your many to many bindings somewhere, meaning you need a third entity class which stores the many-to-many relations.

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;

进行如下课程：

public class ClassA
{
    [Index(IsUnique = true)]
    [MaxLength(200)]
    public String AId { get; set; }
    public virtual List<AB> AB { get; set; }
    public List<ClassC> C { get; set; }
}
public class ClassB
{
    public int BId { get; set; }
    public virtual List<AB> AB { get; set; }
}
public class AB
{
    public int Id { get; set; }
    [Required]
    public virtual A A { get; set; }
    [Required]
    public virtual B B { get; set; }
    [MaxLength(200)]
    public string AId { get; set; }
    public int BId { get; set; }
}
public class ClassC
{
    public int CId { get; set; }
    public ClassA A { get; set; }
    public int AId { get; set; }
}

创建代码优先的迁移，实体框架将对此有所了解并勾画出轮廓您的数据库相应。仅将A和B的ID存储在第三个实体AB中。

Create a code-first migration and entity framework will understand this and sketch out your database accordingly. Store just the IDs of A and B in the third entity AB.

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