3.0.0.rc causes issues with webpacker

[wartron]

Describe the bug
After updating to 3.0.0.rc from 3.0.0.alpha '74c61d42fb9763030aced38af37ba398db3e1c81' /admin route fails with Webpacker::Manifest::MissingEntryError.

1. You want to set webpacker.yml value of compile to true for your environment
   unless you are using the `webpack -w` or the webpack-dev-server.
2. webpack has not yet re-run to reflect updates.
3. You have misconfigured Webpacker's config/webpacker.yml file.
4. Your webpack configuration is not creating a manifest.
Your manifest contains:
{
  "application.js": "/packs/js/application-bfae8bc782a6ad2a86ac.js",
  "application.js.map": "/packs/js/application-bfae8bc782a6ad2a86ac.js.map",
  "entrypoints": {
    "application": {
      "js": [
        "/packs/js/application-bfae8bc782a6ad2a86ac.js"
      ],
      "js.map": [
        "/packs/js/application-bfae8bc782a6ad2a86ac.js.map"
      ]
    }
  }
}

I'm assuming this has something to do #3455

Additional context

• rails version: Rails 6.1.4.4
• rails_admin version: 3.0.0.rc

webpacker.yml

  source_path: app/javascript
  source_entry_path: packs
  public_root_path: public
  public_output_path: packs
  cache_path: tmp/cache/webpacker
  webpack_compile_output: true

  # Additional paths webpack should lookup modules
  # ['app/assets', 'engine/foo/app/assets']
  additional_paths: []

  resolved_paths: ['app/assets']

  # Reload manifest.json on all requests so we reload latest compiled packs
  cache_manifest: false

  # Extract and emit a css file
  extract_css: false

  static_assets_extensions:
    - .jpg
    - .jpeg
    - .png
    - .gif
    - .tiff
    - .ico
    - .svg
    - .eot
    - .otf
    - .ttf
    - .woff
    - .woff2

  extensions:
    - .mjs
    - .js
    - .sass
    - .scss
    - .css
    - .module.sass
    - .module.scss
    - .module.css
    - .png
    - .svg
    - .gif
    - .jpeg
    - .jpg

development:
  <<: *default
  compile: true

  # Reference: https://webpack.js.org/configuration/dev-server/
  dev_server:
    https: false
    host: localhost
    port: 3035
    public: localhost:3035
    hmr: false
    # Inline should be set to true if using HMR
    inline: true
    overlay: true
    compress: true
    disable_host_check: true
    use_local_ip: false
    quiet: false
    pretty: false
    headers:
      'Access-Control-Allow-Origin': '*'
    watch_options:
      ignored: '**/node_modules/**'


test:
  <<: *default
  compile: true

  # Compile test packs to a separate directory
  public_output_path: packs-test

production:
  <<: *default

  # Production depends on precompilation of packs prior to booting for performance.
  compile: false

  # Extract and emit a css file
  extract_css: true

  # Cache manifest.json for performance
  cache_manifest: true

webpack/environment.js

const { environment } = require('@rails/webpacker')

const webpack = require('webpack')
environment.plugins.prepend('Provide',
  new webpack.ProvidePlugin({
    $: 'jquery/src/jquery',
    jQuery: 'jquery/src/jquery'
  })
)


module.exports = environment

[mshibuya]

Recently Webpacker support was introduced by #3414, and it requires some setups which is to be done by installer.
But currently RailsAdmin automatically detects which way to use for asset delivery. In your case, :webpacker was chosen but you haven't run the installer yet. So this is my design mistake.

I'll change the auto-detection until the next release, but for now you can either:

• Run installer to make your RailsAdmin setup Webpacker-ready
• Or continue to use Sprockets by adding config.asset_source = :sprockets to your config/initializers/rails_admin.rb

[wartron]

Thanks @mshibuya

Updating to the latest 'master' and running rails g rails_admin:install added the necessary files and updated the asset_soruce in the config. This is working both local and production environments.