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Please help me understand what heroku run rake assets:precompile exactly does. Ever since I began working on ruby on rails, I would always run these three commands before I push to github and heroku:

bundle exec rake assets:precompile

RAILS_ENV=production bundle exec rake assets:precompile

After I push to heroku, I would run:

heroku run rake assets:precompile

However, when I tried to run it after my last push to heroku, I got a bunch of the same errors on different files. For example:

Warning. Error encountered while saving cache ... can't dump anonymous class ...

To see if I can fix this, I ran

heroku run rake assets:clean and then heroku run rake assets:precompile again. The thing is that everything is working fine, but I just feel iffy having all these warnings/errors. Please help me understand. Thank you!

user3104471
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4 Answers4

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Precompile

To give you some clearer definitions - Heroku isn't the only system which requires you to "precompile" your assets. Asset precompilation is a pre-requisite of most Rails production environments, as it allows you to serve static assets (files) - perfect for speed & efficiency

Here's what the Rails documentation says about it:

In the production environment Sprockets uses the fingerprinting scheme outlined above. By default Rails assumes assets have been precompiled and will be served as static assets by your web server.

During the precompilation phase an MD5 is generated from the contents of the compiled files, and inserted into the filenames as they are written to disc. These fingerprinted names are used by the Rails helpers in place of the manifest name.

The reason why Heroku wants you to precompile your assets is because the Heroku environment is designed for speed & efficiency; and hence does not want to expend CPU power on compiling the assests for each request / instanace of your app

This means you have to either precompile the assets yourself, or let the Heroku buildpacks sort that out for you

Heroku

As mentioned by CWitty, you'll want to make sure you compile your assets locally. And whilst I'm not sure about the errors you've received, I do know one thing: precompilation populates the public/assets folder

This means if you precompile locally before submitting to Heroku, you'll have all your latest assets present in your public/assets directory before you try and run the application on Heroku

Although Heroku does perform precompilation as part of the build process, you'll be much safer (from an exception perspective) by precompiling locally:

$ rake assets:precompile RAILS_ENV=production

This will give you the ability to populate the public/assets folder, allowing you to then push to Heroku without any issues

Richard Peck
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  • so I noticed doing `rake assets:precompile RAILS_ENV=production` and `rake assets:precompile` seem to be mutually exclusive, creating different files. I simply need to do run the production precompile command, then push? I've done that, but assets aren't showing. What else might be the problem? – ahnbizcad Sep 01 '14 at 02:48
  • excellent explanation, btw. Much better than the heroku articles. – ahnbizcad Sep 01 '14 at 02:48
  • What's the difference between `RAILS_ENV=production bundle exec rake assets:precompile` and `rake assets:precompile RAILS_ENV=production`? – ahnbizcad Sep 01 '14 at 02:55
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You should be running this command before you push to Heroku as it **pre**compiles your assets. Heroku will automatically run this command if you are missing a manifast.yml file. After running rake assets:precompile locally you can commit all of the changes and then push to Heroku.

CWitty
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  • Thank you for your answer. Do you have an idea as to why I get the warnings? Are you suggesting I can ignore these? – user3104471 Aug 11 '14 at 23:50
  • I wouldn't ignore them. I just wouldn't run on once your code is on Heroku. I would do it prior to pushing to Heroku. – CWitty Aug 12 '14 at 01:10
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For anyone having trouble figuring out why Heroku won't compile your assets automatically:

If a public/assets/manifest.yml is detected in your app, Heroku will assume you are handling asset compilation yourself and will not attempt to compile your assets. Rails 4 uses a file called public/assets/manifest-.json instead. On both versions you can generate this file by running $ rake assets:precompile locally and checking the resultant files into Git.

I found a sprockets-manifest-*.json and Heroku started compiling my assets automatically after I deleted this file.

In my case, this file was generated by the script rails_composer.

fbelanger
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If you have any .jpeg make sure to change them to .jpg before compiling. The compile step will do it for you but your image_tags will be off if you're specifying your files with extensions.

Jbur43
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