The occurrence and challenge of ActiveRecord lookup tables

I’ve noticed lots of Rails apps end up with database-backed lookup tables. Particularly in systems with some kind of customer or subscription management, it’s almost guaranteed that User or Customer models belong_to SubscriptionLevel or Plan models. Thus, you frequently need to query both models.

If you’re looking for avoidable database work, as I sometimes have, this seems like low-hanging fruit. Plan level models very rarely change. You could replace those Plan or SubscriptionLevel models with a hardcoded data object and move on.

In my experience, you now have a white whale on your hands. This low-hanging fruit may haunt you for a while. It could cause you to invent increasingly implausible mechanisms for ridding yourself of this “technical debt” (scare quotes, it’s a trade-off and not actual technical debt). Teammates will appear drowsy when you mention this problem and its technical details, then back away slowly.

Why is it so tricky to convert AR database lookups to non-AR in-memory lookups?

I’ve attempted this twice. Both times, I tried to grab as little surface area as possible and ended up with nearly all of the models. A current teammate is trying now and suffering a somewhat similar fate. They’re more detail-oriented and motivated than I am, so I hope they’ll succeed. (Ed. they succeeded!)

Is this phenomenon something we can easily write off to coupling or is it something else? My pessimistic, gossip-y sense leads me to think people who have become ORM Skeptic went down this path thinking it’s inevitable if you accept an ORM into your life. They came away a dark shade of who they were with the conclusion that ORMs ruin everything. However, the phases of coping that involve a three thousand word essay and then writing a new database layer thing don’t actually solve this problem.

In the ActiveRecord flavor of ORMs, it is easy to describe model graphs and interactions amongst those graphs. Once you’ve got the whole model graph, its often difficult to isolate a subsection of it. AR, in particular, can make it easy to violate Demeter and reach through that graph in hard-to-refactor ways.

Our lack of great and general tools for working with Ruby code that uses these graphs and rewriting said code is another big challenge. Solving these problems requires visualizing the direct and transitive connections between models. Then you need some kind of refactoring tool to rewrite code to use an indirection object instead of directly coupling. We lack both of those in the Ruby world.

Optimistically, I’d think this is a case of refactoring smarter. Given a solid test suite you could:

  • connect your lookup models to an in-memory SQLite database populated at app start, no need to remove ActiveRecord
  • use one of the several libraries that implement enough of the ActiveRecord interface to replace models with classes backed by static data
  • lots of things I haven't thought or heard of!

The thing you wouldn’t want to do, and where I faltered at least once, was to let it become a long-running task. When you’re making changes all over the code base, any amount of churn behind your back is potentially crippling. If you can freeze the code base, I highly recommend it. (Coincidentally, this is exactly what my smarter-than-me teammate did!)

The other thing to keep in mind is that, inevitably, you will come across weird uses of ActiveRecord and Rails that you didn’t know about, are pretty sure you don’t like, and have to work with anyway. Set aside time for these known unknowns.

When dealing with potentially large, radical changes to your application code, how radical are you willing to go to make many smaller changes than one big one? There’s no crisp answer here. All code grows awkward in different ways. As always: divide, conquer, and celebrate your victories!

Adam Keys @therealadam