Heroku rake aborted

You should get this error on first db:migrate at heroku.

>heroku rake db:migrate
rake aborted!
Please install the postgresql adapter: `gem install activerecord-postgresql-adapter` (pg is not part of the bundle. Add
it to Gemfile.)

Tasks: TOP => db:migrate => db:load_config
(See full trace by running task with --trace)


Adding
group :production do
gem 'pg'
end
to Gemfile solves the problem.