Nested forms for belongs_to relationship - ActiveAdmin

class Product < ActiveRecord::Base
  belongs_to :cost, :class_name => 'Currency', foreign_key: 'cost_id'
  accepts_nested_attributes_for :cost
  attr_accessor :cost_id
end

class Currency < ActiveRecord::Base
  def self.currency_types
    ['SAR','AED','USD','EUR','INR']
  end
end

ActiveAdmin.register Product do
 form do |f|
    f.semantic_errors *f.object.errors.keys
    f.inputs "Details" do
      f.input :name
      f.inputs "cost" do
        f.semantic_fields_for :cost_attributes do |j|
          j.inputs do
            j.input :currency_type, :as => :select, :collection => Currency.currency_types, :label => 'Cost'
            j.input :value
          end
        end
      end
    end
    f.actions
 end

 controller do
   def permitted_params
    params.permit product: [:name, cost_attributes: [:id, :currency_type, :value]]
   end
 end
end