public class Category
{
  public int Id { get; set; }
  public int ParentCategoryId { get; set; }
  public string ChildIDs { get; set; }
}

from cg in db.Categories
           where cg.Id == id
           join urc in
           (from x in db.UrlRecords where x.EntityName == "Category" select x) on cg.Id equals urc.EntityId into urccg
           from f in urccg.DefaultIfEmpty()
           select new
               {
                cg,
                f,
                ParentCategoryId = cg.ParentCategoryId,
                ChildIds = string.Join(",", (from x in db.Categories where x.ParentCategoryId == cg.Id select x.Id))
               }