How to build a feature as a Rails engine in a folder of the main app, loaded as a path gem. Covers isolate_namespace, appending the engine’s migrations to the host instead of copying them, a single lambda hook for auth so the engine never depends on the host, and where engines hurt (specs, Zeitwerk, schema visibility). Real code from a short-link and click-tracking feature.

https://rubyinsights.blog/en/blog/rails-engines-gem-inside-your-app