Ruby on Rails: Can you put Ruby code in a YAML config file?

You can if it is interpreted through ERB, in which case it acts like an ERB view and Ruby code goes between <% and %>

Try:

access_key_id: <%= ENV['S3_KEY'] %>
secret_access_key: <%= ENV['S3_SECRET'] %>

... and see if it works

Using fd.'s example, try swapping out the ERB syntax with string interpolation if your application is configure to use HAML. E.g.,:

access_key_id: #{ENV['S3_KEY']}
secret_access_key: #{ENV['S3_SECRET']}

instead of:

access_key_id: <%= ENV['S3_KEY']} %>
secret_access_key: <%= ENV['S3_SECRET'] %>

Works like a charm for me without any additional code (Rails 4.2):

default: &default
  adapter: <%= 'mysql2' %>

In rails 4.2 using ERB syntax will evaluate the code and return strings.

# environment variables
S3_KEY=01234
S3_SECRET=56789

# yaml file
access_key_id: <%= ENV['S3_KEY'] %>
secret_access_key: <%= ENV['S3_SECRET'] %>

# then you can do
ENV.fetch('access_key_id')
 => "01234"
ENV.fetch('secret_access_key')
 => "56789"

You can also write ruby code in a string in your YAML file and then evaluate it later

# yaml file
retry_interval: '5.minues'

# then you can do
eval(ENV.fetch('retry_interval'))
 => 300 seconds

CAUTION: be very careful when using eval as it can post a serious security risk