Undefined Method Error in ruby on rails, when trying to produce a nested json format using to_json?

This is the product model in my gems lib s4s models folder:

module S4s
   module Models
     class Product < ActiveRecord::Base
       self.table_name = 'product'
       has_many :images, foreign_key: :product_id, class_name: 'S4s::Models::ProductImage'
       has_many :primary_images, -> { where(primary_image: true) }, foreign_key: :product_id, class_name: 'S4s::Models::ProductImage'
       has_one :image, foreign_key: :product_id, class_name: 'S4s::Models::ProductImage'
       has_many :product_images, foreign_key: :product_id, class_name: 'S4s::Models::ProductImage'
       end
   end
end

This is the product_image.rb file in my gems lib s4s models folder:

require 'paperclip'

module S4s
 module Models
  class ProductImage < ActiveRecord::Base
  self.table_name = 'product_images'.freeze

  include S4s::Models::Concerns::Upload

  TYPE_HIGH_RESOLUTION = 'highResolution'
  TYPE_ADDITIONAL      = 'additional'

  IMAGE_VERSIONS = %w|mini small medium xs sm fullxs fullsm large|

  attr_accessor :image_file_size

  alias_attribute :image_file_name, :original_file_name
  alias_attribute :image_content_type, :file_ext
  alias_attribute :image_updated_at, :updated_at

  belongs_to :product, foreign_key: 'product_id'.freeze, class_name: 'S4s::Models::Product'.freeze
  belongs_to :color, foreign_key: 'color_id'.freeze, class_name: 'S4s::Models::Dictionary::Color'.freeze

  validates :title, presence: true

  scope :additional, -> { where(image_type: TYPE_ADDITIONAL) }
  scope :high_resolution, -> { where(image_type: TYPE_HIGH_RESOLUTION) }
  scope :primary_images, -> { where(primary_image: true) }

  after_initialize :set_default_value
  after_save :set_product_colors!

  add_attachment :image,
                 styles:     {
                     mini:   ['100x100#', :jpg],
                     small:  ['220x220#', :jpg],
                     medium: ['380x380#', :jpg],
                     xs:     ['240x240', :jpg],
                     sm:     ['348x348#', :jpg],
                     fullxs: ['480x480#', :jpg],
                     fullsm: ['768x768#', :jpg],
                     large:  ['1000x1000', :jpg],

                 },
                 path_block: -> (style) { self.build_path(style) },
                 matches:    /(png|jpe?g|gif)/i


  # Populate file_name attribute with the current title
  before_image_post_process :set_file_name!

  public

  def url(type = 'mini')
    return nil unless self.product_id.present?

    image.url(type)
  end

  def urls
    Hash[IMAGE_VERSIONS.map { |v| [v, self.url(v)] }]
  end

  def as_json(opts = {})
    {
        id:               self.id,
        is_primary_image: primary_image?,
        product_id:       self.product_id,
        title:            self.title,
        color:            'n/a',
        sku:              self.sku,
        position:         self.position,
        image_type:       self.image_type,
        urls:             self.urls
    }
  end

  def build_path(style)
    return nil if product.nil?

    build_asset_path(style, !(new_system? || title_used?))
  end

  private

  def build_asset_path(style, old_format = false)
    "/products/#{product_id}/#{build_slug(old_format)}-#{style}.#{_find_extension(image_content_type)}"
  end

  def build_slug(old_format)
    if old_format && !file_name.present?
      "#{product.name.parameterize}#{position > 0 ? "-#{position}" : ''}"
    else
      file_name
    end
  end

  def set_product_colors!
    _colors = self.product.images.map(&:color).compact

    if self.product.colors.map(&:id).sort != _colors.map(&:id).sort
      self.product.update_attribute :colors, _colors
    end
  end

  def set_file_name!
    self.file_name  = SecureRandom.hex(20)
    self.new_system = true
  end

  def set_default_value
    self.position   ||= 0
    self.new_system = true if self.new_system.nil?
  end
  end
 end
end

The Logic is that we are calling these models to different apps using S4s::Models::ModelName

Below is the controller file that I am using to render json(this controller is in another app):

class HalfsController < ApplicationController
    def index
      @hotspot = S4s::Models::Product.all
      render json: @hotspot.to_json(include: :product_images)
    end
    ...
end 

I need a nested format of Product_image objects inside product object. I am New to ruby and rails framework, please help me out.

note: I have tried all format to_json such as :product_images and :product_image.. nested is working for many other models in gems but these are not working for product and product_images.. they have used paperclip to upload and generate images url