I have to deal with a large, legacy Rails app, my strategy is to cordon it off and treat it with kid gloves. In this case it's quite literal, my main work machine cannot run a development server or even deploy the codebase. Instead I use a separate laptop. 95% of my time is spent building an external architecture with the eventual goal of pulling out successive pieces of functionality from the main app until nothing is left. When things are planned I push hard for building on the external architecture rather than extending the legacy app.

When I first got to the job I thought it might be worthwhile to refactor and upgrade the old codebase, but eventually settled on the cordon strategy because it's ultimately more flexible, and not having to touch the production site every day gives me a lot of peace of mind.

Anyone in this situation, I would recommend re-implementing your Active Record models with an external database connection using Sequel or even just regular SQL queries. I have a giant query with a crapload of joins that can retrieve product data, it's among the very first things I built at this job.

This provides a nice base to build on, the database is the heart and soul of the business, and it is far easier to work at that level of abstraction rather than trying to mess around inside the codebase.