You'll be looking for a gem called cocoon; you can also watch this Railscast (Nested forms) which is woefully outdated but still explains the structure very well.

The pattern is very simple, but requires some extra parts:

1. Have an ajax button which calls the controller
2. The controller needs to return a form and built fields_for
3. You'll use JS to append the new fields_for to the original form

The biggest problem is the id of your new fields_for - new implementations of this pattern use child_index: Time.now.to_i

I've written about this here.

Here's a new version:

Ajax

Firstly, you need an "Add Qualification" button, which links to your controller through ajax:

#app/views/profiles/_form.html.erb
<%= simple_form_for(@profile) do |f| %>
    <%= f.error_notification %>
    <div class="form-inputs">
       <div class="row">
         <div class="intpol2">Your professional qualifications</div>
         <%= render 'qualifications/form', f: f %>     
       </div>
    </div>
       <%= button_to "+", new_profile_path, method: :get %>
<% end %>

Controller

This will go through the new controller method, which we should be able to manage to return the specific response for the ajax request:

#app/controllers/profiles_controller.rb
class ProfilesController < ApplicationController
   respond_to :js, :html, only: :new
   def new
      @profile = Profile.new
      @profile.qualifications.build
      respond_with @profile #-> will invoke app/views/profiles/new.js.erb
   end
end

Response

Once your new.js.erb has fired, we need to build the new HTML form, extract the fields_for and append it to your view:

#app/views/profiles/new.js.erb
var fields_for = $("<%=j render "profiles/form" %>").html(); //-> might need tweaking
$(fields_for).appendTo("#new_profile.form-inputs");

child_index

You should also change your qualifications/form to include the child_index:

#app/views/qualifications/form.html.erb
<%= simple_fields_for :qualifications, child_index: Time.now.to_i do ...

Child index is meant to denote the index of the fields_for elements. In our case (since we're making all new records), it doesn't matter. Using Time.now.to_i ensures a totally unique id each time.

Finally, you need to make sure you're calling:

 <%= simple_fields_for :qualifications ... %> 

... plural