How do you get the Ruby on Rails generated id of a form element for reference in JavaScript?

I ended up creating a custom form builder to expose the property directly

class FormBuilder < ActionView::Helpers::FormBuilder
  def id_for(method, options={})
   InstanceTag.new( object_name, method, self, object ) \
               .id_for( options )               
  end
end

class InstanceTag < ActionView::Helpers::InstanceTag
  def id_for( options )
    add_default_name_and_id(options)
    options['id']
  end
end

Then set the default form builder

ActionView::Base.default_form_builder = FormBuilder 

Look at the form builder options:

<%= form_for @user do |f| %>
  <% form_css_id = "#" + f.options[:html][:id] %>
<% end %>

Options should at least include the following data: css class, id, http method and authenticity token.

In case someone has a FormBuilder object from a fields_for block, it is possible to get its id using this snippet:

<%= form.fields_for :something do |fields_form| %>
  <%= fields_form.object_name.gsub(/[\[\]]+/, '_').chop %>id
<% end %>

FieldsForm#object_name returns the field's ID as something like this: user[address][0]. Next, the regex substitution changes groups of one or more brackets to underscores. This substitution leaves a trailing underscore, to which it appends the letters id. For the example provided before, this results in user_address_0_id.