complicated problem with javascript.

'm banging with a difficult problem I think.
I have a form like this:

- @company.categorizations.each do |categorization|
          = f.semantic_fields_for :categorizations, categorization do |cat|
              = cat.input :classification, :as => :select, :collection
=> Classification.all,
                                           :member_label =>
:classification_type, :label => false,
                                           :wrapper_html => { :class
=> "company_classification" },
                                           :input_html => { :class =>
"company_select_classification_type" }
                - if categorization.classification
                  = text_field_tag "classification_amount",
                                                            :class =>
                                                            :disabled => true
                - else
                  = cat.input :amount, :label => false, :wrapper_html
=> { :class => "company_classification" },
                                                        :input_html =>
{ :class => "company_input_classification_amount" }

I hava a coffescript code that monitor the change event of the select field.

$(document).on "change", ".company_select_classification_type", ->
    $div = $(this).closest("td").next().find(".company_categorization_amount")
    $.get "/classifications/find_amount",
    id: $(this).val(), (data) ->
      $div.html data

when the select value change I insert in the
".company_categorization_amount" div the code of the file find_amount:

= text_field_tag "classification_amount", @classification.amount,
:class => "company_input_classification_amount".

That work.
If the select value change and it is blank I want to insert
= cat.input :amount.
One for every @company.categorizations.
I don't know how to do this.
Some help?