Phase 7a — Getting Opinionated: Rules-Based Auto-Categorization (and a Seam for the AI Later)

My expense app finally has a point of view on what I'm spending money on. No AI yet — just honest keyword rules, a nullable column, and one interface that means I can bolt an LLM on later without ripping anything out. Here's the build, three "empty value" bugs that bit me, and the habits that kept it clean. Index Where we left off The plan: rules first, AI behind the same door Step 1 — A nullable column (and why nullable matters) Step 2 — The migration: generate → review → apply Step 3 — A dumb-but-working categorize() Step 4 — Wiring it into create (with override precedence) Step 5 — The seam: extracting behind a Categorizer interface Step 6 — The UI loop: show, add, edit 🐛 The war story: three ways "empty" lied to me Thinking like an attacker Learning shortcut vs. production Key habits to keep Next up: Phase 7b Where we left off Phase 6 gave me the receipts — date-range reports and CSV export. I ended that post with a promise: Next up: Phase 7, where categories finally enter the schema and the app starts to get opinionated about what I'm spending on. This is that. But it turned into a bigger beast than one post, so I'm splitting it: Phase 7a (this post): the schema, a rules-based categorizer, the interface seam, and the full UI loop. Phase 7b (next): the actual LLM — an LLMCategorizer that slots in behind the same interface, with caching and a rules fallback. Doing rules first isn't a cop-out. It's the whole strategy. The plan: rules first, AI behind the same door The temptation with "AI categorization" is to reach straight for the API key. I didn't. Here's the order I actually built in, and why: Step What Why this order 1 Nullable category column The app needs somewhere to store a category before it can fill one 2 Rules categorize() A working, free, offline fallback — and a baseline to test against 3 Extract behind an interface So the LLM can slot in later without touching call sites 4 UI loop (show / add / edit) Give the human final say, no matter how smart the