heroku_gemfile.rb

# for Heroku, replace "gem 'sqlite3'" in your Gemfile with this:
group :development, :test do
  # if you already have a 'group :development,:test' block in your
  # Gemfile, you can just move the line "gem 'sqlite3'" into it.
  gem 'sqlite3' # use SQLite only in development and testing
end
group :production do
  gem 'pg' # use PostgreSQL in production (Heroku)
end