item_serializer.rb

class ItemSerializer < ActiveModel::Serializer
  attributes :id,
             :name,
             :description,
             :image_url,
             :created_at,
             :updated_at,
             :order_count

  has_many :order_items
  has_many :orders, through: :order_items

  def order_count
    object.orders.count
  end
end

show.json.jbuilder

json.extract! @item, :id,
                     :name,
                     :description,
                     :created_at,
                     :updated_at

json.order_count @item.orders.count

json.orders @item.orders, :amount,
                          :user_id,
                          :created_at,
                          :updated_at