Fat Rails models aren’t always a sign that we need another architectural layer. Sometimes, we’re missing a domain concept. I explored this idea in a talk at Kaigi on Rails 2026.

Slides: https://decks.yasaichi.com/20261016/domain-modeling-with-vanilla-rails

For example, email sign-up and Google sign-up both create a User, but each has different inputs, validations, and business rules. Instead of just moving those rules into service objects or form objects, we can recognize each sign-up operation as a domain concept and model it as EmailSignUp or GoogleSignUp, while still taking advantage of Rails conventions.

The talk also explores how to discover these missing concepts in an existing codebase.