EF Core 2.1: Self-referencing entity with one to many relationship generates additional column

Question

I have the following entity:

public class Level
{
    public int LevelId { get; set; }
    public int? ParentLevelId { get; set; }
    public string Name { get; set; }

    public virtual Level Parent { get; set; }
    public virtual HashSet<Level> Children { get; set; }   
}

What I am having trouble here, is the Children property, which is configured like this in Fluent API:

modelBuilder.Entity<Level>()
    .HasOne(x => x.Parent)
    .WithMany(x => x.Children)
    .HasForeignKey(x => x.ParentLevelId);

This results in some additional column being added by the migration:

migrationBuilder.AddColumn<int>(
    name: "LevelId1",
    table: "Level",
    nullable: true);

migrationBuilder.CreateIndex(
    name: "IX_Level_LevelId1",
    table: "Level",
    column: "LevelId1");

migrationBuilder.AddForeignKey(
    name: "FK_Level_Level_LevelId1",
    table: "Level",
    column: "LevelId1",
    principalTable: "Level",
    principalColumn: "LevelId",
    onDelete: ReferentialAction.Restrict);

What am I doing wrong here?

Edit: Question was marked as a possible duplicate of this question; however, in that case, the model generation works - the issue is loading the data. Whereas here, the issue is that an additional column is generated.

Answer 1

Something's messed up in your migration. No repro when initializing that model:

using Microsoft.EntityFrameworkCore;
using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;

namespace EfCoreTest
{

    public class Level
    {
        public int LevelId { get; set; }
        public int? ParentLevelId { get; set; }
        public string Name { get; set; }

        public virtual Level Parent { get; set; }
        public virtual HashSet<Level> Children { get; set; }
    }

    public class Db : DbContext
    {
        public DbSet<Level> levels { get; set; }


        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlServer("server=.;database=EfCoreTest;Integrated Security=true");
            base.OnConfiguring(optionsBuilder);
        }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            base.OnModelCreating(modelBuilder);
            modelBuilder.Entity<Level>()
                        .HasOne(x => x.Parent)
                        .WithMany(x => x.Children)
                        .HasForeignKey(x => x.ParentLevelId);

        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            using (var db = new Db())
            {
                db.Database.EnsureDeleted();
                db.Database.EnsureCreated();

                Console.ReadKey();



            }

        }
    }
}

creates table:

CREATE TABLE [levels] (
    [LevelId] int NOT NULL IDENTITY,
    [ParentLevelId] int NULL,
    [Name] nvarchar(max) NULL,
    CONSTRAINT [PK_levels] PRIMARY KEY ([LevelId]),
    CONSTRAINT [FK_levels_levels_ParentLevelId] FOREIGN KEY ([ParentLevelId]) REFERENCES [levels] ([LevelId]) ON DELETE NO ACTION
);

Added a Migration,

PM> Add-Migration InitialCreate

and still no repro:

using Microsoft.EntityFrameworkCore.Metadata;
using Microsoft.EntityFrameworkCore.Migrations;

namespace EfCoreTest.Migrations
{
    public partial class InitialCreate : Migration
    {
        protected override void Up(MigrationBuilder migrationBuilder)
        {
            migrationBuilder.CreateTable(
                name: "levels",
                columns: table => new
                {
                    LevelId = table.Column<int>(nullable: false)
                        .Annotation("SqlServer:ValueGenerationStrategy", SqlServerValueGenerationStrategy.IdentityColumn),
                    ParentLevelId = table.Column<int>(nullable: true),
                    Name = table.Column<string>(nullable: true)
                },
                constraints: table =>
                {
                    table.PrimaryKey("PK_levels", x => x.LevelId);
                    table.ForeignKey(
                        name: "FK_levels_levels_ParentLevelId",
                        column: x => x.ParentLevelId,
                        principalTable: "levels",
                        principalColumn: "LevelId",
                        onDelete: ReferentialAction.Restrict);
                });

            migrationBuilder.CreateIndex(
                name: "IX_levels_ParentLevelId",
                table: "levels",
                column: "ParentLevelId");
        }

        protected override void Down(MigrationBuilder migrationBuilder)
        {
            migrationBuilder.DropTable(
                name: "levels");
        }
    }
}