jquery code to add repeating form parts. Usually for an array of items. Adds a link to add another row.

repeat_last.js.coffee

# Use for serialized Array attributes, to add a link to add rows for it in the form.
# Last child is saved and appended on click.
# .field.repeat_last{data: {'link-name' => 'add item'}}
#   .item
#     = f.text_field :name
#     = f.text_field :url
$(->
  $('form .repeat_last').map(->
    el = $(this)
    last_child = el.find('>*:last-child').clone()
    lnk = $('<a>').html(el.data('link-name') || 'add').click(->
      el.append(last_child.clone())
    )
    el.after(lnk)
  )
)