How to bundler to bundle install mysql2 gem

how_to_install_mysql2_with_bundler.md

So if you have ever run into failing to install mysql2 gem with bundler, here is a quick way to resolve it:

1. Install MySQL with dev headers
2. Configure bundler mysql2.build option: bundle config build.mysql2 --with-mysql-dir=<path_to_mysql_folder>
3. bundle install

If you run into issue like this (which is likely if your MySQL does not get installed into /usr/local/mysql on macOS)

/Users/tle/.asdf/installs/ruby/2.3.5/lib/ruby/gems/2.3.0/gems/mysql2-0.5.2/lib/mysql2.rb:33:in `require':LoadError: dlopen(/Users/tle/.asdf/installs/ruby/2.3.5/lib/ruby/gems/2.3.0/gems/mysql2-0.5.2/lib/mysql2/mysql2.bundle, 9): Library not loaded: /usr/local/mysql/lib/libmysqlclient.20.dylib
  Referenced from: /Users/tle/.asdf/installs/ruby/2.3.5/lib/ruby/gems/2.3.0/gems/mysql2-0.5.2/lib/mysql2/mysql2.bundle

You can resolve it by creating a symlink to that libmysqlclient.20.dylib:

sudo mkdir -p /usr/local/mysql/lib
sudo ln -sf /path/to/mysql/lib/libmysqlclient.20.dylib /usr/local/mysql/lib/libmysqlclient.20.dylib