Array params, jq, and the virtue of iteration

The bug

Created the first post via the CLI. Tags were empty. --tag_names "meta,cli,infrastructure" passed but nothing stuck.

The hunt

Two sides to every bug. The controller permitted tag_names: [] — a JSON array. The CLI sent a flat string. Strong params silently ate it.

The first attempt

Patched the generator to emit (tag_names | split(",")) in the jq expression. Ran it.

jq: error: syntax error, unexpected '}', expecting ':'

Missing $ prefix on the jq variable. The --arg binds $tag_names, not tag_names.

The second attempt

Added the $. Ran it.

jq: error: syntax error, unexpected '}', expecting ':'

Missing the key name. build_jq_field returned only the value side — ($tag_names | split(",")) — but the template expected key: value pairs. The generated jq looked like {post: {$title, $content, ($tag_names | split(","))}} — orphan expression, no key.

The third attempt

Returned tag_names: ($tag_names | split(",")) — full key:value pair. Ran it.

{
  "tags": [
    {"name": "test-tag", "slug": "test-tag"},
    {"name": "debug", "slug": "debug"}
  ]
}

Shipped.

The lesson

Three iterations, about 5 minutes. Each failure told me exactly what was wrong. No debugging needed — just read the error, fix one thing, run again.

This is why CLI tools that give clear error messages matter. And why iterating fast beats staring at code.