How to pre-check checkboxes in formtastic

For multiple check boxes, this way:

<%= f.input :tags, :as => :check_boxes, :collection => some_map.collect { |c| [c[:name], c[:id], {:checked=> tag_ids.include?(c[:id])}] } %>

For anyone else having the same issue:

<%= f.input :some_input, :as => :boolean, :input_html => { :checked => 'checked' } %>