Events

Table of contents

1. Facts, not state
2. Defining an event

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An Event is an immutable record of something that happened. Unlike a traditional model, an event is not current state — it is a fact from history.

Facts, not state

Think about how a typical Rails model works: a User record represents who they are now. When their email changes, you overwrite the old value and it’s gone.

An event works differently. A User::EmailChanged event records what happened — the old email, the new email, and when the change occurred. The fact is stored permanently and can never be updated.

This distinction is the foundation of event sourcing. Your system’s history becomes the source of truth, and current state is derived from it.

Defining an event

Events inherit from Funes::Event and behave like ActiveModel objects. You define their attributes and validations just as you would on any Rails model:

# app/events/debt/issued.rb
module Debt
  class Issued < Funes::Event
    attribute :amount, :decimal
    attribute :interest_rate, :decimal
    attribute :at, :datetime

    validates_presence_of :at
    validates :amount, numericality: { greater_than: 0 }
    validates :interest_rate, numericality: { greater_than_or_equal_to: 0 }
  end
end

Because events are ActiveModel instances and not ActiveRecord models, they are schema-independent. Your historical facts never need a migration just because your UI requirements changed.