class ActionView::Helpers::FormBuilder

def grouped_collection_select(method, collection, group_method, group_label_method, option_key_method, option_value_method, options = {}, html_options = {})

Please refer to the documentation of the base helper for details.

<% end %>
<%= f.submit %>
<%= f.grouped_collection_select :country_id, @continents, :countries, :name, :id, :name %>
<%= form_for @city do |f| %>

Wraps ActionView::Helpers::FormOptionsHelper#grouped_collection_select for form builders: 
def grouped_collection_select(method, collection, group_method, group_label_method, option_key_method, option_value_method, options = {}, html_options = {})
  @template.grouped_collection_select(@object_name, method, collection, group_method, group_label_method, option_key_method, option_value_method, objectify_options(options), @default_html_options.merge(html_options))
end