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WhittakerTech::Midas

Midas is a Rails engine for elegant, multi-currency monetary value management.

Instead of adding price_cents and price_currency columns to every model that needs money, Midas stores all monetary values in a single polymorphic Coin ledger. The Bankable concern gives any ActiveRecord model a clean DSL for declaring and accessing its monetary attributes.

Key capabilities

  • Single canonical ledger — one wt_midas_coins table for every monetary value across your entire application
  • Declarative DSL — has_coin :price / has_coins :subtotal, :tax, :total
  • Exact arithmetic — integer minor-unit math, no floating-point errors
  • Flexible input coercion — accepts Money objects, floats, integers, and strings
  • Token-based formatting — strftime-like grammar for custom display strings
  • RTL support — Unicode bidirectional isolation for Arabic and Hebrew UIs
  • Per-unit pricing — Allocation value object for per-unit / bulk pricing
  • Bank-style currency input — Stimulus controller with automatic decimal handling
  • Additive double-entry bookkeeping — Ledger (accounts + balanced, immutable postings) for consumers needing a full audited trail, since 0.4.0
  • 90%+ test coverage

Requirements

Requirement Version
Ruby >= 3.2
Rails >= 7.1
money gem ~> 6.19

Quick look

class Invoice < ApplicationRecord
  include WhittakerTech::Midas::Bankable

  has_coins :subtotal, :tax, :total
end

invoice = Invoice.create!
invoice.set_subtotal(amount: 100.00, currency_code: 'USD')
invoice.set_tax(amount: 8.50,  currency_code: 'USD')
invoice.set_total(amount: 108.50, currency_code: 'USD')

invoice.subtotal_format   # => "$100.00"
invoice.tax_amount        # => #<Money @fractional=850 @currency="USD">
invoice.total             # => #<WhittakerTech::Midas::Coin ...>

Documentation sections

  • Installation — add the gem, run migrations, register the Stimulus controller
  • Usage — Bankable DSL, Coin arithmetic, Presenter, Allocation, Ledger
  • Architecture — design decisions, module breakdown, schema (including Ledger)
  • API Reference — full method reference; generated API docs at api/WhittakerTech/Midas
  • Examples — e-commerce products, invoicing, bulk pricing, RTL display

Why Midas?

Monetary code is some of the most fragile code in a Rails application. Rounding errors, currency mismatches, and schema duplication accumulate quietly until a financial report or a refund exposes them.

Midas makes the single-ledger approach — standard in accounting software — the default for Rails. One table, one set of rules, one place to add audit hooks or exchange rate logic.