RuntimeError: !!! Missing the mysql2 gem

I started migrating my blog from Rails 2 to Rails 3, working up my deployment setup using rvm and phusion passenger. When setting up the staging server, I run into this error:

RuntimeError: !!! Missing the mysql2 gem. Add it to your Gemfile: gem 'mysql2'

mysql2 gem was already in the Gemfile. After looking around, I found out that I had wrong database adapter because, of course, I have copied the old database.yml from Rails 2 application where is was:

production:
  adapter: mysql
  encoding: utf8
  reconnect: false
  database: blog
  pool: 5
  username: root
  password: 
  socket: /var/run/mysqld/mysqld.sock

The solution is just to change the adapter from mysql to mysql2:

production:
  adapter: mysql2
  encoding: utf8
  reconnect: false
  database: blog
  pool: 5
  username: root
  password: 
  socket: /var/run/mysqld/mysqld.sock