class OrgUnit
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public OrgUnit Parent { get; set; }
        public List<OrgUnit> Children { get; set; }
    }

    class OrgUnitConfig : IEntityTypeConfiguration<OrgUnit>
    {
        public void Configure(EntityTypeBuilder<OrgUnit> builder)
        {
            builder.ToTable("OrgUnit");
            builder.Property(e => e.Name).HasMaxLength(50).IsRequired();

            builder.HasOne(o => o.Parent).WithMany(p => p.Children);
        }
    }

        private static Task ReadOrgUnit(MyDbContext ctx)
        {
            // 使用include()取出root，可以带出直接的children(asia, america)，但是无法带出asia的children。
            //ctx.OrgUnits.Include(o => o.Children).FirstOrDefault(o => o.Parent == null);

            // 反而取出所有的orgunit以后，EF会自动建立连接。
            OrgUnit[] units = ctx.OrgUnits.Where(_ => true).ToArray();

            PrintOrgUnit(units.FirstOrDefault(u => u.Parent == null), 0);
            return Task.CompletedTask;
        }

        private static void PrintOrgUnit(OrgUnit orgUnit, int indent)
        {
            if (orgUnit == null)
                return;

            Console.WriteLine(new string(' ', indent) + orgUnit.Name);
            if (orgUnit.Children == null)
                return;

            foreach (OrgUnit child in orgUnit.Children)
            {
                PrintOrgUnit(child, indent + 2);
            }
        }

        private static async Task SaveOrgUnits(MyDbContext ctx)
        {
            OrgUnit root = new OrgUnit { Name = "root" };
            OrgUnit asia = new OrgUnit { Name = "asia", Parent = root };
            OrgUnit china = new OrgUnit { Name = "china", Parent = asia };
            OrgUnit sg = new OrgUnit { Name = "singapore", Parent = asia };
            OrgUnit america = new OrgUnit { Name = "america", Parent = root };
            OrgUnit usa = new OrgUnit { Name = "usa", Parent = america };
            OrgUnit can = new OrgUnit { Name = "canada", Parent = america };

            ctx.OrgUnits.AddRange(new OrgUnit[] { root, asia, china, sg, america, usa, can });
            await ctx.SaveChangesAsync();
        }