application.js

// This is a manifest file that'll be compiled into application.js, which will include all the files
// listed below.
//
// Any JavaScript/Coffee file within this directory, lib/assets/javascripts, or any plugin's
// vendor/assets/javascripts directory can be referenced here using a relative path.
//
// It's not advisable to add code directly here, but if you do, it'll appear at the bottom of the
// compiled file. JavaScript code in this file should be added after the last require_* statement.
//
// Read Sprockets README (https://github.com/rails/sprockets#sprockets-directives) for details
// about supported directives.
//
//= require rails-ujs
//= require activestorage
//= require turbolinks
//= require_tree .
//= require jquery3
//= require popper
//= require bootstrap-sprockets

$(document).on("turbolinks:load", function() {
  $("form").on("click", ".add_fields", function(event) {
   var regexp, time;
   time = new Date().getTime();
   regexp = new RegExp($(this).data("id"), "g");
   $(".fields").append($(this).data("fields").replace(regexp, time));
   return event.preventDefault() ;
 });
  $("form").on("click", ".remove_record", function(event) {
   $(this).prev("input[type=hidden]").val("1");
   $(this).closest("div.step").hide();
   return event.preventDefault() ;
  });
});

application_helper.rb

module ApplicationHelper
  def link_to_add_row name, form, association, **args
    new_object = form.object.send(association).klass.new
    id = new_object.object_id
    fields = form.fields_for(association, new_object, child_index: id) do |b|
      render(association.to_s.singularize, f: b)
    end

    link_to(name, "#", class: "add_fields " + args[:class],
      data: {id: id, fields: fields.delete("\n")})
  end
end

new.html.erb

<div class="row">
  <div class="col-md-6 offset-md-3">
    <%= form_for @question_bank do |f| %>
      <%= f.label :name, t(".name"), class: "field-label" %>
      <%= f.text_field :name, class: "form-control field-input" %>

      <%= f.label :category, t(".category"), class: "field-label" %>

      <h5>Add questions</h5>

      <div>
        <%= f.fields_for :questions do |ff| %>
          <%= render "question", f: ff %>
        <% end %>
      </div>

      <%= link_to_add_row(t(".add_question"), f, :questions, class: "btn btn-default") %>

      <%= f.submit t(".submit"), class: "btn btn-primary submit-button" %>
    <% end %>
  </div>
<div>

question.html.erb

<div class = "question">
  <%= f.label :content, t(".content_question"), class: "field-label question-label" %>
  <%= f.text_field :content, class: "form-control field-input" %>

  <div>
    <%= f.label :multi_correct, t(".multi"), class: "field-label" %>
    <%= f.check_box :multi_correct, {}, "yes", "no" %>
  </div>

  <%= f.fields_for :answers do |ff| %>
    <%= render "answer", f: ff %>
  <% end %>
</div>