How does Rails identify its current environment?

Sidd · July 18, 2013, 7:03pm

How does rails differentiate between development vs production machine (or does it not) - like when we push to heroku / ec2 / server in grandma’s basement / what have you - How does the app /framework know that “oh, this is the production machine” - and so must look at the production group for the gem file, and for the database.yml, oh and I must also skip the tests, etc. etc.

joel · July 18, 2013, 7:22pm

The app server starting up will pass along the environment it should be running as. By default it assumes “development” unless you tell it otherwise, right?

So - places like heroku will tell it otherwise. Sometimes it’s passed as a flag to the server – for example, with webrick: rails s -e production. Other times it’s an environmental variable: export RAILS_ENV='production' && rails s.

r00k · July 18, 2013, 8:28pm

Hmm.

If only Rails were open-source, we’d be able to examine its code to see how it works!

github.com

rails/rails/blob/main/railties/lib/rails.rb#L52


      
          
          
delegate :initialize!, :initialized?, to: :application
          
          
# The Configuration instance used to configure the Rails environment
          def configuration
            application.config
          end
          
          
def backtrace_cleaner
            @backtrace_cleaner ||= begin
              # Relies on Active Support, so we have to lazy load to postpone definition until Active Support has been loaded
              require "rails/backtrace_cleaner"
              Rails::BacktraceCleaner.new
            end
          end
          
          
# Returns a Pathname object of the current Rails project,
          # otherwise it returns +nil+ if there is no project:
          #
          #   Rails.root
          #     # => #<Pathname:/Users/someuser/some/path/project>

So, based on my reading of the env method, Rails looks for an environment variable of RAILS_ENV, then RACK_ENV, then defaults to “development”.

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How does Rails identify its current environment?

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