Load order of the environment

Rails 3, 4, 5, 6

1. config/application.rb
2. config/environment.rb before the initialize! call (we don't usually edit this file)
3. The current environment, e.g. environments/production.rb
4. Gems
5. Vendored plugins
6. All initializers in config/initializers/*.rb
7. config/environment.rb after the initialize! call (we don't usually edit this file)
8. Your own code from app

Rails 2

1. Code in config/preinitializer.rb (if it exists)
2. environment.rb, code above the Rails::Initializer.run block (put constants here if you want to override them in environments)
3. environment.rb, the Rails::Initializer.run block
4. The current environment, e.g. environments/production.rb
5. Gems
6. Vendored plugins
7. All initializers in config/initializers/*.rb
8. Code below the Rails::Initializer block

Further reading

Consider reading the Rails initialization process guide Show archive.org snapshot for a deep dive into the initialization process.