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

Here is how I fixed this:

 bundle config
 bundle config --delete without
 bundle install --deployment --without development test postgres

Credits: How do you undo bundle install --without