Active Record validation:
new_record?()//用于验证刚新建，但没存入database中的数据 ，返回true或false
persisted?() //和new_record/()正相反，从英文意看出，Returns true if the record is persisted.it's not a new record and it was not destroyed.
create //验证失败仅仅不会执行代码，返回false.create! //带！号的话，验证失败会弹出