Gem::LoadError for mysql2 gem, but it's already in Gemfile

Question

Gem::LoadError
Specified \'mysql2\' for database adapter, but the gem is not loaded.
Add `gem \'mysql2\'` to your Gemfile

This error occurred while

Answer 1

Change to

gem 'mysql2', '~> 0.3.18'

in your Gemfile.

This thread on the official mysql2 Github says to do this. You need to declare that version number if you're rails version 4.x.x.

https://github.com/brianmario/mysql2/issues/675

Then run bundle update mysql2.