Engines and Main App in Rails Apps

My third rails engine added to OppSites brought up some interesting challenges. The first two I installed, from Instacart’s Open Source repository, were completely self contained in their views. This third one, user_impersonate2, was my first experience with an engine that embedded itself within my own layouts.

While that behavior was necessary and useful, it was surprising when I started getting errors saying previously working path helpers were undefined.

It turns out, there is some sort of main_app that can be appended to a path helper to tell it to use the helper in the context of the “main app” instead of the engine. I’m still unclear as to where main_app is defined and how it works exactly, but it does work.

If you were to leave off the main_app “routing proxy” method call, it could potentially go to the engine’s or application’s root, depending on where it was called from.

Ah, so it seems that main_app forces the use application paths rather than the paths from the current context.