Reactive Fields: dependent form fields in plain Ruby

What it does

avo-reactive_fields

Let one field respond to another as your users type. Reactive Fields adds a react_on option to any Avo field, so a city select can repopulate when the country changes, a slug can fill itself from a name, and a price can recompute the moment a quantity is edited, all on the same form. It's all plain Ruby, no JavaScript to write and no Stimulus controller to wire up.

When a watched field changes, the form posts itself back over Turbo and Avo re-renders only the reacting fields. Point a field at one dependency, a list, or every field on the form with react_on: :all, then derive its value with a format_using or options lambda that reads the current @record. Dependent forms like that are fiddly to wire and easy to break on the next Avo upgrade. Here the wiring is already built and vetted, and it moves with Avo instead of drifting.

What you get

Wire a dependent form in one line: add react_on to any field, point it at one field, a list, or :all, and skip the Stimulus controller
Cascading selects that stay in sync: a city dropdown that repopulates the moment the country changes, no stale options
Fields that fill themselves: derive a slug, a total, or a price with a format_using lambda the instant its source changes
Your lambdas always see the form as the user left it, @record carries the latest values on every update, so derived values are never one edit behind
Compare against what changed: original values stay available through Rails' *_was methods
Reacts on the right event for you, selects and checkboxes on change, text inputs on debounced input, so it feels instant without firing on every keystroke

Why it pays off

Ship a dependent form this afternoon with one react_on line, instead of generating and re-reviewing Stimulus and Turbo wiring in every app.
Reactive forms are easy to get subtly wrong: stale @record state, options that do not refresh, fields that fire on the wrong event. Those cases are already covered, vetted across hundreds of apps.
Every Avo release makes it better: the reactive behavior gains improvements and the field options you rely on keep working through upgrades, and fixes reach your app with bundle update.
Tokens, time, and focus go to the form logic, not to front-end plumbing generated from scratch.

Questions

How do I build a dependent select in Rails without JavaScript?

Add react_on to the dependent field. A city select with react_on: :country and an options block that reads @record.country gets a fresh list whenever the country changes. Avo posts the form back over Turbo and re-renders only the fields that react, so there's no Stimulus controller to write.