til/rails/use-ruby-extension-for-template-file.md

Use .ruby Extension For Template File

An interesting feature of Rails that I can't seem to find documented anywhere is that you can write a template file with plain Ruby by using the .ruby extension. For instance, you might want to render some JSON from a template. Instead of using jbuilder or erb, you can have a show.json.ruby file. This is also popular with Turbo Stream files -- e.g. update.turbo_stream.ruby.

How this works is that the entire file is evaluated as if it were a .rb file. Then the return value of the final statement is what is returned and rendered by Rails.

author_byline = @book.authors.map(&:name).to_sentence

data = {
  id: @book.id,
  title: @book.title,
  author: author_byline,
  status: @book.published_at > Time.current ? 'Coming Soon' : 'Published',
  publication_year: @book.published_at.year
}

data.to_json

That final line converts the hash of data that we've built up into a JSON string that can then be rendered by the controller action that corresponds to this view template.

Similarly, you can have a Turbo Stream template show.turbo_stream.ruby that looks something like this:

[
  turbo_stream.prepend("posts", @post),
  turbo_stream.update("form", partial: "form", locals: { post: Post.new })
].join

This template file is made up of a single statement which is an array of turbo stream results that get joined together.