Incorrect simple join table determination in reverse engineering leads to incorrect code generation

[akarboush]

The current simple join table determination in version 6 and 7 does not take into account the join table with referencing forgein key(s), resulting in incorrectly generated code.

6.*

efcore/src/EFCore.Design/Scaffolding/Internal/CSharpDbContextGenerator.cs

Line 1148 in 7d0a885

internal static bool IsManyToManyJoinEntityType(IEntityType entityType)

7.*

efcore/src/EFCore.Design/Extensions/ScaffoldingModelExtensions.cs

Line 22 in 376bd8b

public static bool IsSimpleManyToManyJoinEntityType(this IEntityType entityType)

Microsoft sql db script

CREATE TABLE Blogs (Id int IDENTITY CONSTRAINT [PK_Blogs] PRIMARY KEY,) 
go 

CREATE TABLE Posts (Id int IDENTITY CONSTRAINT [PK_Posts] PRIMARY KEY,) 
go 

CREATE TABLE BlogPosts (
  BlogId int NOT NULL CONSTRAINT [FK_BlogPosts_Blogs] REFERENCES Blogs ON DELETE CASCADE,
  PostId int NOT NULL CONSTRAINT [FK_BlogPosts_Posts] REFERENCES Posts ON DELETE CASCADE,
  CONSTRAINT [PK_BlogPosts ] PRIMARY KEY (BlogId, PostId)
)
go 

CREATE TABLE LinkToBlogPosts (
  LinkId1 int NOT NULL,
  LinkId2 int NOT NULL,
  CONSTRAINT [PK_LinkToBlogPosts] PRIMARY KEY (LinkId1, LinkId2),
  CONSTRAINT [FK_LinkToBlogPosts_BlogPosts] FOREIGN KEY (LinkId1, LinkId2) REFERENCES BlogPosts
) go

code generated

public partial class Blog
{
/* ctor */
    public int Id { get; set; }

    public virtual ICollection<Post> Posts { get; set; }
}

 public partial class Post
{
/* ctor */
    public int Id { get; set; }

    public virtual ICollection<Blog> Blogs { get; set; }
}


 public partial class BlogPost
{
    public int BlogId { get; set; }
    public int PostId { get; set; }

    public virtual LinkToBlogPost LinkToBlogPost { get; set; } = null!;
}

 public partial class LinkToBlogPost
  {
      public int LinkId1 { get; set; }
      public int LinkId2 { get; set; }
  
      public virtual BlogPost Link { get; set; } = null!;
  }



protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Blog>(entity =>
            {
                entity.HasMany(d => d.Posts)
                    .WithMany(p => p.Blogs)
                    .UsingEntity<Dictionary<string, object>>(
                        "BlogPost",
                        l => l.HasOne<Post>().WithMany().HasForeignKey("PostId").HasConstraintName("FK_BlogPosts_Posts"),
                        r => r.HasOne<Blog>().WithMany().HasForeignKey("BlogId").HasConstraintName("FK_BlogPosts_Blogs"),
                        j =>
                        {
                            j.HasKey("BlogId", "PostId").HasName("PK_BlogPosts ");

                            j.ToTable("BlogPosts");
                        });
            });

            modelBuilder.Entity<BlogPost>(entity =>
            {
                entity.HasKey(e => new { e.BlogId, e.PostId })
                    .HasName("PK_BlogPosts ");

// error here
                entity.HasOne() 
                    .WithMany()
                    .HasForeignKey(d => d.BlogId)
                    .HasConstraintName("FK_BlogPosts_Blogs");

// error here
                entity.HasOne() 
                    .WithMany()
                    .HasForeignKey(d => d.PostId)
                    .HasConstraintName("FK_BlogPosts_Posts");
            });

            modelBuilder.Entity<LinkToBlogPost>(entity =>
            {
                entity.HasKey(e => new { e.LinkId1, e.LinkId2 });

                entity.HasOne(d => d.Link)
                    .WithOne(p => p.LinkToBlogPost)
                    .HasForeignKey<LinkToBlogPost>(d => new { d.LinkId1, d.LinkId2 })
                    .OnDelete(DeleteBehavior.ClientSetNull)
                    .HasConstraintName("FK_LinkToBlogPosts_BlogPosts");
            });

            OnModelCreatingPartial(modelBuilder);
        }

As you can see

• A skip navigation for Blog and Post is created despite the BlogPost entity.
• The Blog and Post navigation properties are missing from the BlogPost entity and therefore the navigation expression in the Builder module for the BlogPost is also missing.

I would like to submit a PR fixing the problem as I have already experimented with it locally and fixed it adding a referencing foreign keys check.
Is there anything I should consider before submitting??

[smitpatel]

Referencing FK check shouldn't required ideally. In the presence of FK targeting potential join table, it should create navigations in model which fails navigation check and shouldn't mark entity as join entity. Hence it may not be right fix. Need to investigate why navigations are not assigned for the FK.

[ajcvickers]

Note from triage: it should be possible to work around this issue in EF7 by editing the T4 template used to generate the entity types.

[ErikEJ]

Patch? At least EF Core 8?

[ajcvickers]

@ErikEJ It's been this way since 6, right? If so, then it doesn't meet the bar to patch.

[ErikEJ]

Fair enough. And Power Tools let's you work around it