Examples from Coderwall Protip: Install PostgreSQL 9.2 on OS X Mountain Lion

database.yml

development:
  adapter: postgresql
  encoding: unicode
  host: localhost
  database: myapp_development
  username: <%= ENV["USER"] %>
  password:
  allow_concurrency: true
  pool: 5
  min_messages: warning

test:
  adapter: postgresql
  encoding: unicode
  host: localhost
  database: myapp_test
  username: <%= ENV["USER"] %>
  password:
  allow_concurrency: true
  pool: 5
  min_messages: error

Gemfile

source :rubygems

gem "rails"
gem "pg"
# ...