Many-to-many relationship left and right keys flipped after Entity Framework 5 upgrade

Question

I have some code that saves a many to many relationship in code. It was working fine with Entity Framework 4.1 but after updating to Entity Framework 5, it\'s failing.

Answer 1

I can confirm this is a bug in EF5, although I'm still not sure how it worked in 4.3.1.

The problem is that we are not correctly associating the LeftKey/RightKey calls with their corresponding navigation properties.

I will file an EF6 bug over on our CodePlex project site.

To workaround, I think you will need to either:

1. Configure the association from one side only. Or,
2. Switch the LeftKey/RightKey columns names to be the same in both configurations.

Sorry for the inconvenience.

UPDATE: Here is the bug.

Answer 2

Looking at this I suspect that the problem might be caused by the fact that you map the same relationship twice. And you map it in different order.

I made a simple test where I first mapped the relationship once:

    class Program
{
    static void Main(string[] args)
    {
        Database.SetInitializer(new DropCreateDatabaseAlways<Context>());
        var p = new Parent();
        var c = new Child();
        using (var db = new Context())
        {
            db.Parents.Add(new Parent());
            db.Parents.Add(p);

            db.Children.Add(c);
            db.SaveChanges();
        }

        using (var db = new Context())
        {
            var reloadedP = db.Parents.Find(p.ParentId);
            var reloadedC = db.Children.Find(c.ChildId);

            reloadedP.Children = new List<Child>();
            reloadedP.Children.Add(reloadedC);

            db.SaveChanges();
        }

        using (var db = new Context())
        {
            Console.WriteLine(db.Children.Count());
            Console.WriteLine(db.Children.Where(ch => ch.ChildId == c.ChildId).Select(ch => ch.Parents.Count).First());
            Console.WriteLine(db.Parents.Where(pa => pa.ParentId == p.ParentId).Select(pa => pa.Children.Count).First());
        }
    }
}

public class Parent
{
    public int ParentId { get; set; }
    public ICollection<Child> Children { get; set; }

}

public class Child
{
    public int ChildId { get; set; }
    public ICollection<Parent> Parents { get; set; }
}

public class Context : DbContext
{
    public Context() : base("data source=Mikael-PC;Integrated Security=SSPI;Initial Catalog=EFTest")
    {

    }

    public IDbSet<Child> Children { get; set; }
    public IDbSet<Parent> Parents { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
        modelBuilder.Entity<Child>()
            .HasMany(x => x.Parents)
            .WithMany(x => x.Children)
            .Map(c =>
            {
                c.MapLeftKey("ChildId");
                c.MapRightKey("ParentId");
                c.ToTable("ChildToParentMapping"); 
            });

    }
}

And then I changed the OnModelCreating to be:

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
        modelBuilder.Entity<Child>()
            .HasMany(x => x.Parents)
            .WithMany(x => x.Children)
            .Map(c =>
            {
                c.MapLeftKey("ChildId");
                c.MapRightKey("ParentId");
                c.ToTable("ChildToParentMapping"); 
            });

        modelBuilder.Entity<Parent>()
           .HasMany(x => x.Children)
           .WithMany(x => x.Parents)
           .Map(c =>
           {
               c.MapLeftKey("ParentId");
               c.MapRightKey("ChildId");
               c.ToTable("ChildToParentMapping");
           });
    }

What I found and suspected is that the first run generates this sql:

exec sp_executesql N'insert [dbo].[ChildToParentMapping]([ChildId], [ParentId])
values (@0, @1)
',N'@0 int,@1 int',@0=1,@1=2

In constrast to the second which generates:

exec sp_executesql N'insert [dbo].[ChildToParentMapping]([ParentId], [ChildId])
values (@0, @1)
',N'@0 int,@1 int',@0=1,@1=2

You see the values flipped? Here it actually count the ChildId column as ParentId. Now this doesn't crash for me but I let EF create the database which means it probably just switch the column names and if I would look at the foreign keys they would be switched too. If you created the database manually that probably won't be the case.

So in short: You mappings aren't equal and I expect one of them to be used and that one is probably wrong. In earlier verions I guess EF picked them up in different order.

UPDATE: I got a bit curious about the foreign keys and checked the sql.

From the first code:

ALTER TABLE [dbo].[ChildToParentMapping] ADD CONSTRAINT [FK_dbo.ChildToParentMapping_dbo.Children_ChildId] FOREIGN KEY ([ChildId]) REFERENCES [dbo].[Children] ([ChildId]) ON DELETE CASCADE

And from the second code:

ALTER TABLE [dbo].[ChildToParentMapping] ADD CONSTRAINT [FK_dbo.ChildToParentMapping_dbo.Children_ParentId] FOREIGN KEY ([ParentId]) REFERENCES [dbo].[Children] ([ChildId]) ON DELETE CASCADE

Now that is not nice. ParentId mapped against Children is certainly not what we want.

So is the second mapping wrong? Not really because see what happend when I removed the first one:

ALTER TABLE [dbo].[ChildToParentMapping] ADD CONSTRAINT [FK_dbo.ChildToParentMapping_dbo.Parents_ParentId] FOREIGN KEY ([ParentId]) REFERENCES [dbo].[Parents] ([ParentId]) ON DELETE CASCADE

Somehow having two mappings seems to mess things up. Bug or not I don't know.