- form_for @user , '/demo1', :id => 'demo-fields-for' do |f|
  = f.text_field :gender
  - fields_for @user.permission do |permission|
    = permission.check_box :can_edit
    = permission.check_box :can_delete
  - f.fields_for :telephone do |child_form|
    = child_form.label :number
    = child_form.text_field :number
  # Iterates over every address instance in the collection and yields
  - f.fields_for :addresses do |child_form|
    = child_form.label :name
    = child_form.text_field :name
    - unless child_form.object.new_record?
      = child_form.check_box '_destroy'
      = child_form.label '_destroy', :caption => 'Remove'