After reading the answer which suggested I use Simple_form gem with bootstrap integration, I installed it and created my form according to simple_form instructions, but the inpu
You should try the following:
<%= form_for("user", :url => main_app.user_registration_path, :html => { :class => "form-horizontal" } ) do |f| %>
Note that the bootstrap uses some specific selectors for some classes / html elements, so that if you forget to add an element or a class, everything else will be messed up... In this aspect there's no leeway.
On a side note, you should definitely try simple_form, and the bootstrap integration. It will make your life easier.
Update:
<%= simple_form_for("user", :url => main_app.user_registration_path, :html => { :class => "form-horizontal" } ) do |f| %>
<% end %>