Entity Framework relation not loaded from database
ado.net c# database entity-framework entity-framework-6
Question
When using Entity Framework 6 Code First to experiment, I run into the issue of a many-to-many connection that is not properly synced to my "internal" objects.
I took the 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; }
}
In contrast to A and C, who are in a one-to-many connection, A and B are many to many. I create the mappings using Fluent API.
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");
}
Invoking SaveChanges after creating objects of classes ClassA, ClassB, and ClassC in my database context (). The values are, in my view, accurately saved in the database:
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, the relationships are not successfully restored if I run my test programme again.
This implies:
- There are no objects in the B's collection in ClassA. (and vice versa).
- Only if I had already accessed the object of ClassC via my database context could I access it through ClassA.
Is the model definition incorrect, or do I need to manually initiate the synchronisation?
I appreciate it.
Accepted Answer
Your class structures are flawed. If you follow logic, you must store your many-to-many bindings someplace, which necessitates the use of a third entity class to hold the many-to-many relations.
using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
Create your classes as follows:
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 take this into account while laying up your database. Just save A and B's IDs in the third entity, AB.
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