gitlab-ce/doc/development/rails_initializers.md

Rails initializers

By default, Rails loads Zeitwerk after the initializers in config/initializers are loaded. Autoloading before Zeitwerk is loaded is now deprecated but because we use a lot of autoloaded constants in our initializers, we had to move the loading of Zeitwerk earlier than these initializers.

A side-effect of this is that in the initializers, config.autoload_paths is already frozen.

To run an initializer before Zeitwerk is loaded, you need put them in config/initializers_before_autoloader. Ruby files in this folder are loaded in alphabetical order just like the default Rails initializers.

Some examples where you would need to do this are:

1. Modifying Rails' config.autoload_paths
2. Changing configuration that Zeitwerk uses, e.g. inflections