I'm building a Rails engine that lets SaaS customers ask questions about their own data in plain language. I don't want an LLM writing SQL or having direct access to my database. Not in a production SaaS application. This is why, and what this series will build, test and measure.

Context

My last series, DataPorter, was about engineering a gem: a DSL, an orchestrator, generators, a UI. This one has a second goal. I want to learn LLM integration and retrieval (RAG) properly, at a low level, with numbers instead of vibes.

So here is the deal. I'm building a real gem, in the open, and the series follows the order in which I solve the problems. Each part covers code that exists and is tested, every decision comes with the options I turned down, and once the LLM is in, the numbers come from evaluation runs, not from impressions.

The problem

Picture a SaaS that sells order management to online shops.…