Migrate to Webpacker an instance app

In order to migrate an existing Decidim app to Webpacker, there are a few changes your need to run in your code.

this recipe might not work for all the cases, it tries to cover the generic scenarios. If you find yourself with a complex scenario and want to improve this guide feel free to open a PR to complete the guide.

About Webpacker

It’s recommended to understand how Webpacker works. More information:


Before starting the migration, please check you have the following dependencies installed:

  • Node.js version 16.9.x (this version is mandatory)

  • Npm version 7.21.x (it works with other versions, but this is the recommended)

  • Decidim updated to at least v0.25.0.rc4


Add Webpacker to the application

Follow these steps. If you want more information you can find it at the Webpacker installation guide.

  • Install it

bundle exec rails webpacker:install
  • Install Decidim webpacker custom code

bundle exec rails decidim:webpacker:install

This task do a few steps to allow the Rails instance to have a webpacker instance sharing the code between itself and Decidim gem.

This task is automatically performed every time decidim is updated, to get the latest Webpack configuration. This happens when you run the decidim:upgrade task.

Copy Decidim custom CSS and Javascript

webpacker:install task should have created to you the following dirs structure:

  ├── entrypoints
  └── src
  └── stylesheets
  └── images

If it’s not the case you must create it. Then, copy Decidim customizable assets

These files are hooked into Decidim packs (specifically into decidim-core pack) and will be automatically included inside that pack when compiled.

You can download these files directly with these commands:

mkdir -p app/packs/src/decidim app/packs/stylesheets/decidim app/packs/images
touch app/packs/src/decidim/.keep app/packs/stylesheets/decidim/.keep app/packs/images/.keep
wget https://raw.githubusercontent.com/decidim/decidim/develop/decidim-generators/lib/decidim/generators/app_templates/decidim_application.js -O app/packs/src/decidim/decidim_application.js
wget https://raw.githubusercontent.com/decidim/decidim/develop/decidim-generators/lib/decidim/generators/app_templates/decidim_application.scss -O app/packs/stylesheets/decidim/decidim_application.scss
wget https://raw.githubusercontent.com/decidim/decidim/develop/decidim-generators/lib/decidim/generators/app_templates/_decidim-settings.scss -O app/packs/stylesheets/decidim/_decidim-settings.scss

Migrate images

Copy the existing images from app/assets/images to app/packs/images. Images will be available at /media/images/image.png

cp -rp app/assets/images/* app/packs/images/

For instance, add the image "app/packs/images/test.png" in a view using the helper:

<%= image_pack_tag "media/images/test.png" %>

Note that, by default, all images are flattened in a single directory (any subdirectory structure is lost when requiring an image using the image_pack_tag). So, if you have a file in a subdirectory such as app/packs/images/subfolder/test.png, you still need to call for the image using image_pack_tag "media/images/test.png"

Migrate stylesheets

Existing stylesheets should be moved from app/assets/stylesheets to app/packs/stylesheets and imported with @import into decidim_application.scss. Take into account that you might need to adjust a bit the paths of the @import to adjust them to the new locations.

If that CSS file is replacing a Decidim file, there’s no need to add it to decidim_application.scss.

If any of the CSS is re-defining CSS vars add them to _decidim-settings.scss.

Migrate javascripts

Existing javascripts should be moved from app/assets/javascsripts to app/packs/src and imported with import into decidim_application.js. Take into account that you might need to adjust a bit the paths of the import to adjust them to the new locations.

If that JS file is replacing a Decidim file, there’s no need to add it to decidim_application.js

Update Rails helpers

javascript_include_tag and stylesheet_link_tag have been replaced by javascript_pack_tag and stylesheet_pack_tag. This only needs to be done if you’re modifying the application.html file or partial in your layout.

For images, if they are in app/packs/images you need to replace ìmage_tag with image_pack_tag.

Migrate vendorized files and gems

Sometimes assets are included in vendor/assets/ folder or imported from gems. For each specific one you should check:

  1. if the asset is a javascript that is available as npm package the recommendation is to add it to package.json with npm install. If it’s not available you might want to copy it to app/packs/src and import it.

  2. if the asset is a stylesheet it should be copied to app/packs/stylesheets and imported with @import…​ from _decidim-settings.scss. Alternatively you can use the optional asset includes as described below to include these in the Decidim main SCSS files.

Migrate uploads to ActiveStorage

The old method (CarrierWave) to handle user uploads has changed, now it uses ActiveStorage. This means that all the old files need to be migrated to the new system. Use the following rake task to perform this task:

RAILS_ENV=production bin/rails decidim:active_storage_migrations:migrate_from_carrierwave_to_active_storage

Optional asset includes

Decidim Webpacker provides a new configuration convention that allows you to add extra configurations for webpacker using a configuration file named config/assets.rb. Within this file, you have the following API methods available:

# frozen_string_literal: true
# This file is located at `config/assets.rb` of your module.

# Define the base path of your module. Please note that `Rails.root` may not be
# used because we are not inside the Rails environment when this file is loaded.
base_path = File.expand_path("..", __dir__)

# If you want to import some extra SCSS files in the Decidim main SCSS file
# without adding any extra stylesheet inclusion tags, you can use the following
# method to register the stylesheet import for the main application. This would
# include an SCSS file at `app/packs/stylesheets/your_app_extensions.scss` into
# the Decidim's main SCSS file.

# If you want to do the same but include the SCSS file for the admin panel's
# main SCSS file, you can use the following method.
Decidim::Webpacker.register_stylesheet_import("stylesheets/your_app_admin_extensions", group: :admin)

Remove Sprockets references

The completely remove Sprockets references from your application:

  • Review your Gemfile and remove any reference to sprockets and sassc-rails

  • Remove config/initializers/assets.rb

  • Remove app/assets folder

  • In config/application.rb replace:

require 'rails/all'


require "decidim/rails"
# Add the frameworks used by your app that are not loaded by Decidim.
# require "action_cable/engine"
# require "action_mailbox/engine"
# require "action_text/engine"
  • In config/environments/.rb remove any line containing config.assets. (i.e config.assets.debug = true)

Help Decidim to know the application’s assets folder

To prevent Zeitwerk issues trying to autoload the non-ruby application folders, modify the config/initializers/decidim.rb file to include the following:

# Inform Decidim about the assets folder
Decidim.register_assets_path File.expand_path("app/packs", Rails.application.root)


The deployment needs to be updated to manually run npm install before assets are precompiled.

In the case of Capistrano this can be done with a before hook (can be added at the end of your config/deploy.rb file):

namespace :deploy do
  desc "Decidim webpacker configuration"
  task :decidim_webpacker_install do
    on roles(:all) do
      within release_path do
        execute :npm, "install"

  before "deploy:assets:precompile", "deploy:decidim_webpacker_install"

Also, in the case of Capistrano it’s interesting to add to the shared_paths the following folders:

  • storage IMPORTANT!

  • tmp/webpacker-cache

  • node_modules

  • public/decidim-packs


If you have the following exception when executing bundle exec rails decidim:upgrade or bundle exec rails decidim:webpacker:install

npm ERR! code ERESOLVE
npm ERR! ERESOLVE unable to resolve dependency tree
npm ERR!

Then you need to check again that you’re using the correct Node.js and NPM versions.

In some case it might be that some packages are not well resolved when installing NPM modules. If you get messages like:

[webpack-cli] Error: Cannot find module '@rails/webpacker'

These might be due some kind of corruption in your package-json.lock file, you can try either to remove this file and recreate it with npm install or to add the package @rails/webpacker manually in your package.json file next to the @decidim/webpacker package:

    "@decidim/webpacker": "^0.25.2",
    "@rails/webpacker": "^6.0.0-rc.5",

Then execute npm install.