Rails Conventions

This skill only covers what an agent got wrong without it in baseline tests. Everything else is standard Rails 8 practice, which the agent already follows. Where a project's own code or docs/conventions/ disagrees, follow the project.

Rules

• Flash wording: notice: 'Successfully Created Widget', 'Successfully Updated Widget', 'Successfully Deleted Widget'. Use alert: for failures.

• Failed saves: render :new, status: :unprocessable_content, never :unprocessable_entity, which is the deprecated name for the same 422.

• Forms: always simple_form_for and simple_fields_for, never form_with or form_for. Non-model forms use a symbol: simple_form_for :search, url: widgets_path, method: :get, which nests params under :search.

• Route concerns pass the parent type with parent_resource.name.classify, not a hard-coded string or a concern option:

  concern :commentable do
    resources :comments, commentable_type: parent_resource.name.classify
  end

  The controller turns it back into the record with resource_for(:commentable_type) from rolemodel_rails, called only on collection actions. Read polymorphic-parent-resources before writing that controller.

• Dynamic forms (dependent selects, conditional fields) use the turbo_form gem. Read dynamic-forms.

• Mounted engines that expose data or controls (Sidekiq Web, PgHero, Blazer) go behind an admin-only route constraint, and dev-only engines (Letter Opener Web) mount only outside production. This one is a safety rule, kept even though the baseline tests didn't cover it.

Gotchas

• params.expect needs double brackets for nested attributes. parts_attributes: [[:id, :name, :_destroy]] matches the indexed hash that fields_for submits. The single-bracket form parts_attributes: %i[id name _destroy] silently drops every nested record under expect, though it works under permit.
• Lexxy rich-text fields report edits with a lexxy:change event. Anything that reacts to form edits by listening for change or input (auto-save, unsaved-changes warnings, dynamic form triggers) must listen for lexxy:change too, or edits in the editor are missed.