Simple Form: Non object related attribute. How to?
<%= simple_form_for @study,:url => studies_path, :html => { :class => 'form-horizontal' } do |f| %>
    <%= f.input :category, :collection => @categories, :label => "Category", :selected => @categories.first %>
  <%= f.button :submit, t('add_form'),:class => 'btn-primary' %>

<%= simple_form_for @study,:url => studies_path, :html => { :class => 'form-horizontal' } do |f| %>
  <%= f.input :category, :collection => @categories, :label => "Category", :selected => @categories.first %>
  <%= text_field_tag :blah %>
  <%= f.button :submit, t('add_form'),:class => 'btn-primary' %>
<% end %>