Example of how you can use ruby classes to abstract away from using entirely active record objects.

item.rb

module Purchase
  class Item
    attr_reader :student, :item

    delegate :cost, to: :item

    def initialize(student, item)
      @student = student
      @item = item
    end

    def purchase
      if ::PurchaseCalc.new.possible?(student.points, cost)
        point_ledger = ::PointLedger.new(student)
        point_ledger.deduct(cost, item)
        process
        save_objects
      end
    end

    private
    def process
    end

    def save_objects
      ::ActiveRecord::Base.transaction do
        student.student_profile.save
        student.save
      end
    end
  end
end

souvenir.rb

module Purchase
  class Souvenir < Item
  end
end

trip.rb

module Purchase
  class Trip < Item
    def process
      student.current_location = item.name
    end
  end
end