How do I use the Bootstrap form select css with a Rails model?

First of all, for bootstrap's form-control class to work, you must add it to select tag itself

Per apidoc of rails form select

select(object, method, choices = nil, options = {}, html_options = {}, &block)

you can add html class as

<%= f.select(:ampm, options_for_select([['AM', 1], ['PM', 2]]), {}, {class: "form-control"}) %>

or simply

<%= f.select :ampm, [['AM', 1], ['PM', 2]], {}, {class: "form-control"}) %>

This works for me:

<%= f.select :secretquestion, options_for_select([
  ["Select One", ""], 
  "Your nick name", 
  "your schoool name", 
  "your love name", 
  "your comapnay name", 
  "your favourite website"]), 
  {}, { class: "form form-group form-control" } %>