So you got this error, even though your Gemfile bundles mysql2:
!!! Missing the mysql2 gem. Add it to your Gemfile: gem 'mysql2'
or
Please install the mysql adapter: `gem install activerecord-mysql-adapter` (mysql is not part of the bundle. Add it to Gemfile.)
The reason for this confusing error message is probably that your Gemfile says mysql2, but your database.yml still uses the mysql adapter. Change it to use the mysql2 adapter:
development:
adapter: mysql2
database: myproject_development
encoding: utf8
username: root
password: secret
Note if you're on an old Rails version
If you're using a Rails version less than 3.1 (e.g. 2.3.x, 3.1.x), you need the mysql2 in a 0.2.x version. Newer versions no longer include the ActiveRecord adapter (which is part of ActiveRecord since Rails .1). So your Gemfile should say:
gem 'mysql2', '~0.2.0'
Posted by Henning Koch to makandra dev (2012-08-07 13:47)