如果记录存在于实体框架中，则不要插入子表使用id

Question

我有注册和部门表。

Registration Table:

     public int ID { get; set; }
            public string Username { get; set; }
            public Nullable<int> DepartmentID { get; set; }
     public virtual Department Department { get; set; }

Department Table:
  public int ID { get; set; }
        public string Name { get; set; }

我试图插入注册表和子表部门。但是重复插入了。如果部门名称已存在，我希望将部门表中的ID插入注册。

Registration regEntity = new Registration();
            regEntity.ID = reg.ID;
           regEntity.Username = reg.Username;
           regEntity.Email = reg.Email;
          regEntity.Department = new Department { Name = reg.Department };



 WorkOrderDBEntities db = new WorkOrderDBEntities();
                db.Registration.Add(registration);
                db.SaveChanges();