Add new fields in order's line items

order_contents_decorator.rb

#This is the last file where you need to change
module Spree
	OrderContents.class_eval do 

    def initialize(order)
      @order = order
    end

		def add(variant, quantity = 1, options = {}, attributes_name)
      timestamp = Time.current
      line_item = add_to_line_item(variant, quantity, options, attributes_name)
      options[:line_item_created] = true if timestamp <= line_item.created_at
      after_add_or_remove(line_item, options)
    end

    def remove(variant, quantity = 1, options = {}, attributes_name)
      line_item = remove_from_line_item(variant, quantity, options, attributes_name)
      after_add_or_remove(line_item, options)
    end 

    private

    def add_to_line_item(variant, quantity, options = {}, attributes_name)
      line_item = grab_line_item_by_variant(variant, false, options)

      if line_item
        line_item.quantity += quantity.to_i
        line_item.currency = currency unless currency.nil?
      else
        opts = { currency: order.currency }.merge ActionController::Parameters.new(options).
                                            permit(PermittedAttributes.line_item_attributes)
        line_item = order.line_items.new(quantity: quantity,
        																  attributes_name: attributes_name,
                                          variant: variant,
                                          options: opts)
      end
      line_item.target_shipment = options[:shipment] if options.has_key? :shipment
      line_item.save!
      line_item
    end

    def remove_from_line_item(variant, quantity, options = {}, attributes_name)
      line_item = grab_line_item_by_variant(variant, true, options)
      line_item.quantity -= quantity
      line_item.target_shipment= options[:shipment]

      if line_item.quantity.zero?
        order.line_items.destroy(line_item)
      else
        line_item.save!
      end

      line_item
    end

	end
end

orders_controller_decorator.rb

#After adding permitted attributes you need to add that value into your orders controller
def populate
  order    = current_order(create_order_if_necessary: true)
  variant  = Spree::Variant.find(params[:variant_id])
  quantity = params[:quantity].to_i
  options  = params[:options] || {}
  attributes_name = params[:attributes_name]
  if quantity.between?(1, 2_147_483_647)
    begin
      order.contents.add(variant, quantity, options, attributes_name)
    rescue ActiveRecord::RecordInvalid => e
      error = e.record.errors.full_messages.join(", ")
    end
  else
    error = Spree.t(:please_enter_reasonable_quantity)
  end

  if error
    flash[:error] = error
    redirect_back_or_default(spree.root_path)
  else
    respond_with(order) do |format|
      format.html { redirect_to cart_path }
    end
  end
end

spree.rb

#After adding a field in database you need to permit this attribute to do that you need to add the following line in your 
#config/initializer/spree.rb file
Spree::PermittedAttributes.line_item_attributes.push :attributes_name