Entity Framework relation not loaded from database

Question

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!

The code you use to materialize your objects from the DB may be useful here. Depending on context configuration it may be an issue of using .Include(). — Jonathan Shay, Sep 03 '17 at 02:40

score 4 · Answer 1 · answered Nov 15 '18 at 17:40

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.

Afnan Makhdoom · Accepted Answer · 2017-09-03T07:50:59.557

0

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.

edited Sep 03 '17 at 07:50

answered Sep 03 '17 at 03:23

Afnan Makhdoom

654
1
8
20

Is such a link class really necessary in my object-based model? Because of the correct results in the database, I thought Entity Framework would create such a link class internally as a result of the "HasMany()..WithMany()" specification in the OnModelCreating method. I was the opinion that I do not have to define such link classes explicitly in my object-based model and Entity Framework handles this class transparently. – PraMiD Sep 03 '17 at 07:57
1

Trust me, I thought the same when I started, unfortunately, it doesn't. This is in fact the right approach. – Afnan Makhdoom Sep 03 '17 at 07:58
OK. The data is loaded into memory if I use this approach. However there is still the problem of lazy loading. EF only adds entries to the AB list of ClassA and ClassB I have explicitly accessed manually using the DbSet of my context. Is there some command like: "Load everything into memory"? – PraMiD Sep 04 '17 at 09:53
I did not specify the entries as virtual and disabled lazy loading in the configuration of my context – PraMiD Sep 04 '17 at 10:03
You might also want to check in on the `Include` method. I suggest you see my answer [here](https://stackoverflow.com/questions/46006859) – Afnan Makhdoom Sep 05 '17 at 21:52
Using the `Include()` method would be possible, but I wanted to hide database access from the user of internal classes. What I want to do: 1. Get an object of `ClassA` using the `Find()` method of my `DbSet` 2. Automatically get the `List` filled. PS: This does not work the other way round, too. I.E. if I want to access A through an object of `ClassC`, it is null if I did not query the ClassA's object manually beforehand. PSS: All properties have the `virtual` keyword now – PraMiD Sep 08 '17 at 09:15

Entity Framework relation not loaded from database

2 Answers2

Linked