When using the Ruby buildpack, this checks to see whether assets were compiled locally and committed to git for each deploy by looking for a
manifest.yml file in the repo. This is shown by the following message in the build log:
Detected manifest file, assuming assets were compiled locally
This usually indicates that
rake assets:precompile has been run locally and then those files were committed to git. Heroku interprets this to mean that you'd rather precompile your assets manually, which some users like to do for faster builds. See https://devcenter.heroku.com/articles/rails-asset-pipeline#compiling-assets-locally
To stop this happening you need to remove the
public/assets, commit that change to git and then redeploy. Rails 4 uses a file called
public/assets/manifest-<md5 hash>.json instead. More recent versions use
public/assets/.sprockets-manifest-<md5 hash>.json (note the dot
. indicating it may be hidden by default on your machine).
Once removed, ensure that you've committed the change to git and deploy again.