Entity Framework relation not loaded from database

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.

I got following classes:

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 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");
}

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 |
+-----+----------+

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

This means:

• The B's collection in ClassA does not contain any object (and vice versa).
• I can only access the object of ClassC through ClassA if I accessed this object beforehand through my database context.

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

Thanks in advance!

回答1:

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;

Make your classes like below:

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; }
}

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.

回答2:

For anyone facing the same problem than me, my issue was that I was missing the word virtual in the related entity. As far as I know, virtual is needed when Lazy Loading is enabled, this helps Entity Framework to load the navigation property correctly. Hope it helps.

来源：https://stackoverflow.com/questions/46019705/entity-framework-relation-not-loaded-from-database