gitlab-org--gitlab-foss/doc/development/rails_initializers.md

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# 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`
1. Changing configuration that Zeitwerk uses, for example, inflections

## Database connections in initializers

Ideally, database connections are not opened from Rails initializers. Opening a
database connection (for example, checking the database exists, or making a database
query) from an initializer means that tasks like `db:drop`, and
`db:test:prepare` will fail because an active session prevents the database from
being dropped.

To help detect when database connections are opened from initializers, we now
warn in `STDERR`. For example:

```shell
DEPRECATION WARNING: Database connection should not be called during initializers (called from block in <module:HasVariable> at app/models/concerns/ci/has_variable.rb:22)
```

If you wish to print out the full backtrace, set the
`DEBUG_INITIALIZER_CONNECTIONS` environment variable.