The Webpack file loader copies referenced files to the destination output directory, with an MD5 hash. The other term for this is a "digest".
By default the filename of the resulting file is the MD5 hash of the file's contents with the original extension of the required resource.
The most common use cases for Webpack processed files are images used for backgrounds in CSS and fonts for CSS. However, this applies to any file that might be processed using the Webpack file loader.
To understand the problem, it helps to read this article:
What is fingerprinting and why should I care
Basically, when Rails prepares assets for production deployments, it also adds a digest
to the file names. E.g., img1.jpg becomes img1-dbu097452jf2v2.jpg.
When compiling its native css Rails transforms all urls and links to digested
versions, i.e. background-image: image-url(img1.jpg) becomes
background-image: url(img1-dbu097452jf2v2.jpg). However this doesn't happen for js and
css files compiled by webpack on the client side, because they don't use
image-url and asset-url. Without some fix, these assets would fail to load.
When Webpack's client JavaScript uses images in render methods, e.g. <img src='...' /> or
in css, e.g. background-image: url(...) The code (such as the CSS) generated by the Webpack
will have the Webpack digested name (MD5 hash). Since the Webpack generated CSS expects
just one level of "digesting", this "double-digesting" from Rails will cause such these assets
fail to load.
If you are interested in learning how to use assets in your React components, read this doc: Webpack, the Asset Pipeline, and Using Assets w/ React.
React on Rails creates symlinks of non-digested versions (original webpack digested file names)
to the Rails deployed digested versions when doing a Rails assets compile. The solution is
implemented using assets:precompile after-hook in
file lib/tasks/assets.rake
The assets for symlinking are defined by config.symlink_non_digested_assets_regex in
config/initializers/react_on_rails.rb.
To disable symlinks set this parameter to nil.
If you run
cd spec/dummy
RAILS_ENV=production bundle exec rake assets:precompile
rails s -e production
You will see this. This shows how the file names output by rails. Note the original names after being processed by Webpack are just MD5's.
I, [2016-07-17T23:46:56.301981 #77382] INFO -- : Writing /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/server-bundle-42935dea382802a27e91b7df444a2813f74b4e6a0fce5606d863aaa10c0623d7.js
I, [2016-07-17T23:46:56.305649 #77382] INFO -- : Writing /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/server-bundle-42935dea382802a27e91b7df444a2813f74b4e6a0fce5606d863aaa10c0623d7.js.gz
I, [2016-07-17T23:46:56.370390 #77382] INFO -- : Writing /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/application_static-dfa728160c3cdebc633c2f6fb3823411530b307044f4dfe460790eef00b4e421.js
I, [2016-07-17T23:46:56.370566 #77382] INFO -- : Writing /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/application_static-dfa728160c3cdebc633c2f6fb3823411530b307044f4dfe460790eef00b4e421.js.gz
I, [2016-07-17T23:46:56.372895 #77382] INFO -- : Writing /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/application_static-17ed778d5061d4797556632b7bfbf405e067d9e7f140060a7f56a09788251f16.css
I, [2016-07-17T23:46:56.373012 #77382] INFO -- : Writing /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/application_static-17ed778d5061d4797556632b7bfbf405e067d9e7f140060a7f56a09788251f16.css.gz
I, [2016-07-17T23:46:56.374531 #77382] INFO -- : Writing /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/2ac2dd94f9b7e54292f6d051f1e4e756-ab14eebb171a9a5c9bfdeb2f88933d2dc4904ea8bb09444984e52b13d230e251.svg
I, [2016-07-17T23:46:56.374818 #77382] INFO -- : Writing /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/2ac2dd94f9b7e54292f6d051f1e4e756-ab14eebb171a9a5c9bfdeb2f88933d2dc4904ea8bb09444984e52b13d230e251.svg.gz
I, [2016-07-17T23:46:56.392207 #77382] INFO -- : Writing /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/5cf5db49df178f9357603f945752a1ef-033650e1d6193b70d59bb60e773f47b6d9aefdd56abc7ccdba3c7bed4e57ccad.png
I, [2016-07-17T23:46:56.393208 #77382] INFO -- : Writing /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/8970f5e1e92aea933b502a2d73976b76-877bde3739dc7080c3fb00ee9012db6f21ed0dbbf11cd596dbb6e1a35bfb71f9.png
I, [2016-07-17T23:46:56.395490 #77382] INFO -- : Writing /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/ecb4572a5e478b107dfcb60c16a7eefa-6d1ab3741d5a164dc2aab48bb74429aebe2e2e29606feca581081697624dc18c.ttf
I, [2016-07-17T23:46:56.395846 #77382] INFO -- : Writing /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/ecb4572a5e478b107dfcb60c16a7eefa-6d1ab3741d5a164dc2aab48bb74429aebe2e2e29606feca581081697624dc18c.ttf.gz
I, [2016-07-17T23:46:56.396979 #77382] INFO -- : Writing /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/fbd0d00cc9b670f05c17893a40da08d0-5731789fd0d7847a582b52b55a83e7a0ad4684acd5a9b487557635a08c112d0e.svg
I, [2016-07-17T23:46:56.397669 #77382] INFO -- : Writing /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/fbd0d00cc9b670f05c17893a40da08d0-5731789fd0d7847a582b52b55a83e7a0ad4684acd5a9b487557635a08c112d0e.svg.gz
I, [2016-07-17T23:46:56.399261 #77382] INFO -- : Writing /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/fc2dcaaf2057331ff76c5d37e1aa7056-efba50c701b697fc8160603b9e876adcf47511f35af68701db285272c965a45f.svg
I, [2016-07-17T23:46:56.399660 #77382] INFO -- : Writing /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/fc2dcaaf2057331ff76c5d37e1aa7056-efba50c701b697fc8160603b9e876adcf47511f35af68701db285272c965a45f.svg.gz
React On Rails: Symlinking /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/2ac2dd94f9b7e54292f6d051f1e4e756-ab14eebb171a9a5c9bfdeb2f88933d2dc4904ea8bb09444984e52b13d230e251.svg to /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/2ac2dd94f9b7e54292f6d051f1e4e756.svg
React On Rails: Symlinking /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/2ac2dd94f9b7e54292f6d051f1e4e756-ab14eebb171a9a5c9bfdeb2f88933d2dc4904ea8bb09444984e52b13d230e251.svg.gz to /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/2ac2dd94f9b7e54292f6d051f1e4e756.svg.gz
React On Rails: Symlinking /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/5cf5db49df178f9357603f945752a1ef-033650e1d6193b70d59bb60e773f47b6d9aefdd56abc7ccdba3c7bed4e57ccad.png to /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/5cf5db49df178f9357603f945752a1ef.png
React On Rails: Symlinking /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/8970f5e1e92aea933b502a2d73976b76-877bde3739dc7080c3fb00ee9012db6f21ed0dbbf11cd596dbb6e1a35bfb71f9.png to /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/8970f5e1e92aea933b502a2d73976b76.png
React On Rails: Symlinking /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/ecb4572a5e478b107dfcb60c16a7eefa-6d1ab3741d5a164dc2aab48bb74429aebe2e2e29606feca581081697624dc18c.ttf to /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/ecb4572a5e478b107dfcb60c16a7eefa.ttf
React On Rails: Symlinking /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/ecb4572a5e478b107dfcb60c16a7eefa-6d1ab3741d5a164dc2aab48bb74429aebe2e2e29606feca581081697624dc18c.ttf.gz to /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/ecb4572a5e478b107dfcb60c16a7eefa.ttf.gz
React On Rails: Symlinking /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/fbd0d00cc9b670f05c17893a40da08d0-5731789fd0d7847a582b52b55a83e7a0ad4684acd5a9b487557635a08c112d0e.svg to /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/fbd0d00cc9b670f05c17893a40da08d0.svg
React On Rails: Symlinking /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/fbd0d00cc9b670f05c17893a40da08d0-5731789fd0d7847a582b52b55a83e7a0ad4684acd5a9b487557635a08c112d0e.svg.gz to /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/fbd0d00cc9b670f05c17893a40da08d0.svg.gz
React On Rails: Symlinking /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/fc2dcaaf2057331ff76c5d37e1aa7056-efba50c701b697fc8160603b9e876adcf47511f35af68701db285272c965a45f.svg to /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/fc2dcaaf2057331ff76c5d37e1aa7056.svg
React On Rails: Symlinking /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/fc2dcaaf2057331ff76c5d37e1aa7056-efba50c701b697fc8160603b9e876adcf47511f35af68701db285272c965a45f.svg.gz to /Users/justin/shakacode/react_on_rails/spec/dummy/public/assets/fc2dcaaf2057331ff76c5d37e1aa7056.svg