manage_to_many_checkbox.rb

# Adapted from http://github.com/marcomd/Logbook/blob/e3537af3170442b0a2cf93a5060c645643b88446/app/helpers/application_helper.rb

# usage example
# = manage_to_many_checkbox(@projects, @user, "project", 3)

# add to application_helper.rb
  def manage_to_many_checkbox(collection, object, str_object_many, cols=1)
    html = ""
    i=0
    cb_style = cols == 1 ? '' : 'float:left;width: #{100/cols}%;'
    collection.each do |item|
      id_html = "#{str_object_many}_#{item.id.to_s}"
      fld_cnt = content_tag(:span,
                  check_box_tag("#{str_object_many}_ids[]",
                            item.id,
                            object.send("#{str_object_many}s").include?(item),
                            {:id => id_html}) +
                  label_tag(id_html, item.name), 
                  :style => cb_style)
      if i % cols == 0 && cols > 1
        html << content_tag(:div, fld_cnt, :style => 'display: block')
      elsif i % cols == 0 && cols == 1
        html << fld_cnt
        html << tag(:br)
      else
        html << fld_cnt
      end
      i+=1
    end
    html << content_tag(:div, '', :class => 'clear')
    raw html
  end