validate, validate_length_of

http://api.rubyonrails.org/classes/ActiveRecord/Validations/ClassMethods.html#M001328
validates_length_of :content, :in => 1..300, :on => :create, :if => lambda{ |article| article.user_id != 1 }

:if 只有在article.user_id 不等於 1時, 才會有這樣的validate,
:on 在create的時候