## database.yml
development:
  username: devc0_us3r
  password: s3cr3t_osit099
  database: devc0_development

test:
  username: devc0_us3r
  password: s3cr3t_osit099
  database: devc0_test

production:
  username: <%= ENV[ 'DB_USERNAME' ] %>
  password: <%= ENV[ 'DB_PASSWORD' ] %>
  database: <%= ENV[ 'DB_NAME' ] %>

## connection.rb
config = YAML.load_file( 'database.yml' )
# This a hash with the config vars.
# e.g.: [ 'development' => { 'username' => '...' }, 'test' => { 'username' => '...' }, 'production' => { 'username' => '...' } ]
# So, given you have the environment variable set to development, you want to do something like:
connection = MySQL.connecition.new( config[ 'development' ][ 'username' ], config[ 'development' ][ 'password' ] )
# Or if you want to simplify that procces:
env = 'development' || 'test' || 'production'
config = YAML.load_file( 'database.yml' )[ env ]
connection = MySQL.connecition.new( config[ 'username' ], config[ 'password' ] )