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

It doesn't load mysql2 gem because new version of mysql2 (>= 0.4.0) gem unable to load the mysql2_adaptor. Can you try this?

gem 'mysql2', '~> 0.3.13'

Hopefully, it should work.