Dynamic update nested fields_for

Question

I've been struggling with this issue for weeks and haven't been able to find or figure out a solution. Here is what I'm trying to accomplish: a form that is automatically populated with a list of questions based on user input. Specifically, I have a list of all possible questions (PossibleQuestion model) that I want to use to populate the form based on the site a user selects.

I know I'm not rendering a proper response to the ajax request. I'm currently getting an undefined local variable or method f error. I found several SO questions regarding passing the form builder (here and here), however none of them have helped.

I'm still new to ruby and rails, so any help or direction is greatly appreciated!

Versions (ruby 2.3, rails 4.2.2). Also using SimpleForm and Cocoon gems.

report.rb

class Report < ActiveRecord::Base
  belongs_to :site
  has_many :questions, :dependent => :destroy
  accepts_nested_attributes_for :questions, :allow_destroy => true
end

site.rb

class Site < ActiveRecord::Base
  has_many :reports
  validates :state, presence: true
end

question.rb

class Question < ActiveRecord::Base
    belongs_to :report
    has_many :answers, :dependent => :destroy
    accepts_nested_attributes_for :answers, :allow_destroy => true
end

possible_question.rb

class PossibleQuestion < ActiveRecord::Base
  validates :content, :state, presence: true
end

answer.rb

class Answer < ActiveRecord::Base
  belongs_to :question
end

reports_controller.rb

def new
    @report = Report.new
    @questions = PossibleQuestion.all
    @questions.each do |q|
        question = @report.questions.build
        question.content = q.content
        question.answers.build
    end
end

def questions_list
  @site = Site.find(params[:id])
  @state = @site.state
  @questions = PossibleQuestion.where(state: @state).to_a
  respond_to do |format|
    format.js
  end
end

report.js

$(document).ready(function() {
    $('#report_site_id').change(function() {
        site_id = $('#report_site_id').find(':selected').val();
        $.ajax({
                url: '/questions_list',
                data: { id: site_id, },

questions_list.js.erb

$(".questions").html("<%= j render 'question_fields', :locals => { :questions => @questions } %>");

I've tried passing the formbuilder object f here in locals, however it doesn't work. I get an error that f is undefined.

_question_fields.html.erb

<%= f.simple_fields_for :questions do |builder| %>
  <%= f.label f.object.content %>
  <%= f.hidden_field :content, :value => f.object.content %>

  <%= f.simple_fields_for :answers do |builder| %>
    <%= render 'answer_fields', :f => builder %>
  <% end %>
<% end %>

_form.html.erb

<%= simple_form_for @report do |f| %>
  <%= f.association :site %>

  <div class="questions"></div>
<% end %>