问在.NET核心2.0中自我引用许多类而不返回集合？

EN

public class ProductCategory
        {
            public int ID { get; set; }
            public string Name { get; set; }

            [DataType(DataType.Html)]
            public string Description { get; set; }

            public int DisplayOrder { get; set; }

            public int MenuIconXPosition { get; set; }
            public int MenuIconYPosition { get; set; }

            public virtual ICollection<ProductSubCategory> ParentCategory { get; set; }
            public virtual ICollection<ProductSubCategory> ChildCategory { get; set; }

            public ICollection<ProductProductCategory> ProductProductCategories { get; set; }
        }

public class ProductSubCategory
    {
        public int ChildCategoryId { get; set; }
        public virtual ProductCategory ChildCategory { get; set; }

        public int ParentCategoryId { get; set; }
        public virtual ProductCategory ParentCategory { get; set; }
     }

    modelBuilder.Entity<ProductSubCategory>()
        .HasKey(p => new { p.ChildCategoryId, p.ParentCategoryId });
    modelBuilder.Entity<ProductSubCategory>()
        .HasOne(p => p.ParentCategory)
        .WithMany(p => p.ChildCategory)
        .HasForeignKey(p => p.ParentCategoryId);
    modelBuilder.Entity<ProductSubCategory>()
        .HasOne(p => p.ChildCategory)
        .WithMany(p => p.ParentCategory)
        .HasForeignKey(p => p.ChildCategoryId);

    var result = _context.ProductCategories
        .AsNoTracking()
        .Include(i => i.ParentCategory)
        .Include(i => i.ChildCategory)
        .OrderBy(i => i.DisplayOrder);