different text_fields for different selected item

Hell:
  When i use select tag in a form,how to show different text_fields
below select tag for different selected item without having to click a
submit tag.I must know
how to write javascript to accomplish it or there is a rails way to do
it?
My second question is:after showing different text_fields for different
selected item,how to manage the database side?Different table for
different text_fields resulted from selected item or only one table for
all these text_fields despite of the selected item?
I have looked for it in rails doc,but did not get an answer.It is
really a headache for me.

anybody can help me?

Guo Yangguang wrote:

Guo Yangguang wrote: