linq to entity 插入失败了

public bool SetEbookTags(Book book, string tags)
        {
            bool returnval = false;
            if (tags != null && tags.Length > 0)
            {
                string[] tagstr = tags.Split(',');
                for (int i = 0; i < tagstr.Length; i++)
                {
                    try
                    {
                        int id = Convert.ToInt32(tagstr[i]);
                        BookTag bk = db.BookTags.SingleOrDefault(d => d.ID == id);
                        if (bk != null)
                        {
                            book.BookTags.Add(bk);
                            Save();
                            returnval = true;
                        }
                        else
                            break;
                    }
                    catch { }
                }
            }
            return returnval;
        }

这些代码无法实现把BookTag插进Book表中，不知为何，调了好久，恼火啊