expense.rb

class Expense < ActiveRecord::Base
  composed_of :amount, :class_name => "Money", :mapping => [ %w(amount_in_cents cents)], :converter => lambda { |amount| amount.to_money }
end

migration.rb

class CreateExpenses < ActiveRecord::Migration
  def self.up
    create_table :expenses do |t|
      t.integer :amount_in_cents
      t.timestamps
    end
  end

  def self.down
    drop_table :expenses
  end
end

usage.rb

# even though amount_in_cents is stored in the database, you access the the amount
# by the amount method.
expense = Expense.create! :amount => "$5.00"
expense.amount