Rails ActionText: support prism syntax highlight code blocks. And transform <action-text-attachment/> to <img/> to work with any frontend.

action_text_parser.rb

# lib/action_text_parser.rb
# require 'action_text_parser' in application.rb
class ActionTextParser
  def self.run(action_text_record)
    html = action_text_record.body.to_html
    nodes = Nokogiri::HTML::DocumentFragment.parse html
    nodes = self.parse_attachments(nodes)
    nodes = self.parse_code(nodes)
    nodes.to_html
  end
  
  def self.parse_attachments(nodes)
    # transform every <action-text-attachment/> to <img/>
    nodes.css('action-text-attachment').map do |attachment|
      src = attachment.attributes['url'].value
      width = attachment.attributes['width'].value
      height = attachment.attributes['height'].value
      alt = attachment.attributes['caption'].try(:value) || attachment.attributes['filename'].try(:value)
      attachment.swap("<img src='#{src}' width='#{width}' height='#{height}' alt='#{alt}' title='#{alt}' />")
    end
    nodes
  end

  def self.parse_code(nodes)
    # transform <pre>...</pre> to "<pre><code class='#{lang-xxx}'>...</code></pre>"
    nodes.css('pre').map do |pre|
      lang_class = pre.content.split("\n")[-1].strip
      if lang_class.present? && lang_class[/lang\-*/].present?
        pre.swap("<pre><code class='#{lang_class}'>#{pre.content.strip.gsub(lang_class, '')}</code></pre>")
      end
    end
    nodes
  end
end

posts_controller.rb

class Api::V1::PostsController < ApplicationController
  def show
    @post = Post.find_by_id(params[:id])
    if @post
      render json: { content_html: ActionTextParser.run(@post.content) }
    end
  end
end

show.html.erb

<!-- /posts/show.html.erb -->
<link href="https://cdnjs.cloudflare.com/ajax/libs/prism/1.17.1/themes/prism-okaidia.min.css" rel="stylesheet" />
<link href="https://cdnjs.cloudflare.com/ajax/libs/prism/1.17.1/plugins/line-numbers/prism-line-numbers.min.css" rel="stylesheet" />
<script src="https://cdnjs.cloudflare.com/ajax/libs/prism/1.17.1/components/prism-core.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/prism/1.17.1/plugins/autoloader/prism-autoloader.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/prism/1.17.1/plugins/line-numbers/prism-line-numbers.min.js"></script>
<%= raw ActionTextParser.run @post.content %>

Nice! Glad to see my post could be helpful to someone else. Thanks for tagging.