Campo do form has many through não aparece (Utilizando a gem wicked)

add_services.slim

.row.wrapper.border-bottom.white-bg.page-heading
  .col-lg-12
    h1 Ordem de Serviços

.wrapper.wrapper-content.animated.fadeInRight					
	.row
		.col-lg-12
			.ibox.float-e-margins
				= simple_form_for @service_order, method: :put, url: wizard_path, html: { id: 'form' } do |f|

					= f.input :cost, as: :hidden, input_html: { value: @service_order.invoice_sub_total }

					.ibox-title
						h3 Selecione selecione o(s) serviço(s):
					.ibox-content
            .form-inputs
								= f.simple_fields_for :service_order_services do |ff|
									= ff.association :service
                  
						.form-actions
							= f.button :submit, 'Salvar e avançar'

			
			.ibox.float-e-margins
			    .ibox-content
						= link_to raw('<i class="fa fa-angle-double-left"></i> Voltar'), service_orders_path, class: 'btn btn-default'
						= link_to 'Avançar', next_wizard_path, class: 'btn btn-primary'

Gemfile

source 'https://rubygems.org'

# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '5.0.1'
gem 'bootstrap-sass', '~> 3.3.7'
gem 'font-awesome-rails', '~> 4.7', '>= 4.7.0.1'

gem 'jquery-rails', '~> 4.2', '>= 4.2.2'

# Use SCSS for stylesheets
gem 'sass-rails', '~> 5.0', '>= 5.0.6'

# Use Uglifier as compressor for JavaScript assets
gem 'uglifier', '~> 3.0', '>= 3.0.4'

# Use CoffeeScript for .js.coffee assets and views
gem 'coffee-rails', '~> 4.2', '>= 4.2.1'

# Use Puma as the app server
gem 'puma', '~> 3.0'

# See https://github.com/sstephenson/execjs#readme for more supported runtimes
# gem 'therubyracer', platforms: :ruby

# Turbolinks makes following links in your web application faster. Read more: https://github.com/rails/turbolinks
gem 'turbolinks', '~> 5.0', '>= 5.0.1'

# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '~> 2.6', '>= 2.6.1'

gem 'simple_form', '~> 3.4', github: 'plataformatec/simple_form', branch: 'master'
gem 'slim-rails', '~> 3.1', '>= 3.1.1'
gem 'country_select', '~> 3.0'

gem 'rails-jquery-tokeninput', '~> 0.2.6'

gem 'cocoon', '~> 1.2', '>= 1.2.9'

gem 'devise', '~> 4.2'

gem 'pundit', '~> 1.1'

gem 'brdinheiro', '~> 3.3'
gem 'brdata', '~> 3.3'
gem 'validates_cpf_cnpj', '~> 0.2.0'

gem 'wicked', '~> 1.3'

group :development, :test do
  # Call 'byebug' anywhere in the code to stop execution and get a debugger console
  gem 'byebug', platform: :mri
  # Use sqlite3 as the database for Active Record
	gem 'sqlite3', '~> 1.3', '>= 1.3.13'
end

group :development do
  # Access an IRB console on exception pages or by using <%= console %> anywhere in the code.
  gem 'web-console', '>= 3.3.0'
  gem 'listen', '~> 3.0.5'
  # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
  gem 'spring'
  gem 'spring-watcher-listen', '~> 2.0.0'
end

group :production do
	gem 'pg', '~> 0.18.4'
  gem 'rails_12factor', '~> 0.0.3'
  gem 'newrelic_rpm', '~> 3.15', '>= 3.15.2.317'
end

ruby "2.4.0"

service.rb

class Service < ApplicationRecord
  
	has_many :service_order_services
	has_many :service_orders, through: :service_order_services
  
end

service_order.rb

class ServiceOrder < ApplicationRecord

  include ActionView::Helpers::NumberHelper
	
	has_many :service_order_services, inverse_of: :service_order
	has_many :services, through: :service_order_services, :class_name => 'Service'
	
  accepts_nested_attributes_for :services
	accepts_nested_attributes_for :service_order_services

end

service_order_service.rb

class ServiceOrderService < ApplicationRecord
  
  belongs_to :service
  belongs_to :service_order
  accepts_nested_attributes_for :service

end

service_orders_controller.rb

class ServiceOrdersController < ApplicationController
  before_action :set_service_order, only: [:show, :edit, :update, :destroy, :invoice_print]
  before_action :authenticate_user!

  # GET /service_orders
  # GET /service_orders.json
  def index
    @service_orders = ServiceOrder.all
  end

  # GET /service_orders/1
  # GET /service_orders/1.json
  def show
  end

  def invoice_print
    render :layout => "empty"
  end

  # GET /service_orders/new
  def new
    @service_order = ServiceOrder.new
    @service_order.build_production
    @service_order.service_order_services.build
  end

  # GET /service_orders/1/edit
  def edit
  end

  # POST /service_orders
  # POST /service_orders.json
  def create
    @service_order = ServiceOrder.new
    @service_order.save(validate: false)
    redirect_to service_order_step_path(@service_order, ServiceOrder.form_steps.first)
  end

  # PATCH/PUT /service_orders/1
  # PATCH/PUT /service_orders/1.json
  def update
    respond_to do |format|
      if @service_order.update(service_order_params)
        format.html { redirect_to service_orders_url, notice: 'Service order was successfully updated.' }
        format.json { render :show, status: :ok, location: @service_order }
      else
        format.html { render :edit }
        format.json { render json: @service_order.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /service_orders/1
  # DELETE /service_orders/1.json
  def destroy
    @service_order.destroy
    respond_to do |format|
      format.html { redirect_to service_orders_url, notice: 'Ordem de serviço excluída com sucesso.' }
      format.json { head :no_content }
    end
  end

  private
    # Use callbacks to share common setup or constraints between actions.
    def set_service_order
      @service_order = ServiceOrder.find(params[:id])
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def service_order_params
      params.require(:service_order).permit(
        :description, :comments, :discount, :cost, :status, :client_id,
        production_attributes: [ :id, :date_end ],
        service_order_product_services_attributes: [ :id, :_destroy, :product_service_id, product_service_attributes: [ :id, :_destroy, :serial_number ]],
        service_order_services_attributes: [ :id, :_destroy, :service_id, service_attributes: [ :id, :_destroy ]],
        service_order_products_attributes: [ :id, :_destroy, :product_id, product_attributes: [ :id, :_destroy ]],
        product_services_attributes: [ :id, :_destroy, :name, :serial_number]
      )
    end
end

steps_controller.rb

class ServiceOrder::StepsController < ApplicationController
  include Wicked::Wizard
  steps *ServiceOrder.form_steps

  def show
    @service_order = ServiceOrder.find(params[:service_order_id])
    @service_order.service_order_services.build
    @service_order.build_production unless @service_order.production.present?
    render_wizard
  end

  def update
    @service_order = ServiceOrder.find(params[:service_order_id])
    @service_order.update(service_order_params(wizard_value(step)))
    render_wizard @service_order
  end

  def finish_wizard_path
    @service_order
  end

  private

    def service_order_params(step)
      permitted_attributes = case wizard_value(step)
                             when :add_client
                               [ :client_id ]
                             when :add_product_services
                               [ service_order_product_services_attributes: [ :id, :_destroy, :product_service_id, product_service_attributes: [ :id, :_destroy, :serial_number ]]]
                             when :add_services
                               [ service_order_services_attributes: [ :id, :_destroy, :service_id, service_attributes: [ :id, :_destroy ]]]
                             when :add_products
                               [ service_order_products_attributes: [ :id, :_destroy, :product_id, :amount, product_attributes: [ :id, :_destroy ]]]
                             when :add_infos
                               [ :description, :comments, :discount, :cost, :status, production_attributes: [ :id, :date_end ]]
                             end

      params.require(:service_order).permit(permitted_attributes).merge(form_step: wizard_value(step))
    end

end