This gem helps you integrate Money fields into your ActiveRecord models without having to add new columns each time.

It also helps manage and sum various components of your models to keep amount definitions consistent across your application.

Add

gem 'amountable', github: 'instacart/amountable'

to your Gemfile. Then run

bundle

and

rake amountable:install:migrations

and finally

rake db:migrate

Setup your model

class Order < ActiveRecord::Base
  act_as_amountable
  amount :subtotal, sets: [:total]
  amount :delivery_fee, sets: [:total, :fees]
  amount :bags_fee, sets: [:total, :fees]
  amount :sales_tax, sets: [:total, :taxes]
  amount :local_tax, sets: [:total, :taxes]
end

act_as_amountable can take the storage option:

  act_as_amountable storage: :table

where storage can be either :table, the amounts table will be used to store amounts, or :jsonb, a JSONB field will be used on the amountable object to store amounts are json. If you use the JSONB format, you can specify the name of the column with the column option. The default value for the storage option is :table.

then create it

order = Order.create(
  subtotal: Money.new(123),
  delivery_fee: Money.new(100),
  bags_fee: Money.new(10),
  sales_tax: Money.new(56)
  )
order.subtotal
# => #<Money fractional:123 currency:USD>
order.total
# => #<Money fractional:289 currency:USD>
order.fees
# => #<Money fractional:110 currency:USD>
order.taxes
# => #<Money fractional:56 currency:USD>

When you run the migration, an amounts table will be created with a polymorphic relationship to an amountable, which in the above example would be your orders.

When the an order is created with some amounts, the associated Amount objects are persisted.

Amount objects are persisted in bulk, so there are no N + 1 queries. If an amount is zero, no Amount model is created.

If you choose the JSONB storage option, the amounts table will not be used. Instead a JSONB column on the target model will be used.