rails g controllerが上手くできない

Question

rails g controllerが成功しない理由がよくわかりません、教えてほしいです！

maikit:~/projects/pictweet $ rails g controller tweets
/usr/local/rvm/gems/ruby-2.3.0/gems/activerecord-4.2.2/lib/active_record/connection_adapters/connection_specification.rb:177:in `rescue in spec': Specified 'mysql2' for database adapter, but the gem is not loaded. Add `gem 'mysql2'` to your Gemfile (and ensure its version is at the minimum required by ActiveRecord). (Gem::LoadError)
        from /usr/local/rvm/gems/ruby-2.3.0/gems/activerecord-4.2.2/lib/active_record/connection_adapters/connection_specification.rb:174:in `spec'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/activerecord-4.2.2/lib/active_record/connection_handling.rb:50:in `establish_connection'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/activerecord-4.2.2/lib/active_record/railtie.rb:120:in `block (2 levels) in <class:Railtie>'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/activesupport-4.2.2/lib/active_support/lazy_load_hooks.rb:38:in `instance_eval'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/activesupport-4.2.2/lib/active_support/lazy_load_hooks.rb:38:in `execute_hook'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/activesupport-4.2.2/lib/active_support/lazy_load_hooks.rb:45:in `block in run_load_hooks'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/activesupport-4.2.2/lib/active_support/lazy_load_hooks.rb:44:in `each'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/activesupport-4.2.2/lib/active_support/lazy_load_hooks.rb:44:in `run_load_hooks'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/activerecord-4.2.2/lib/active_record/base.rb:316:in `<module:ActiveRecord>'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/activerecord-4.2.2/lib/active_record/base.rb:26:in `<top (required)>'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/activesupport-4.2.2/lib/active_support/dependencies.rb:274:in `require'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/activesupport-4.2.2/lib/active_support/dependencies.rb:274:in `block in require'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/activesupport-4.2.2/lib/active_support/dependencies.rb:240:in `load_dependency'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/activesupport-4.2.2/lib/active_support/dependencies.rb:274:in `require'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/spring-1.7.2/lib/spring/application.rb:338:in `active_record_configured?'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/spring-1.7.2/lib/spring/application.rb:259:in `disconnect_database'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/spring-1.7.2/lib/spring/application.rb:97:in `preload'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/spring-1.7.2/lib/spring/application.rb:143:in `serve'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/spring-1.7.2/lib/spring/application.rb:131:in `block in run'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/spring-1.7.2/lib/spring/application.rb:125:in `loop'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/spring-1.7.2/lib/spring/application.rb:125:in `run'
        from /usr/local/rvm/gems/ruby-2.3.0/gems/spring-1.7.2/lib/spring/application/boot.rb:19:in `<top (required)>'
        from /usr/local/rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
        from /usr/local/rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
        from -e:1:in `<main>'

Answer 1

Specified 'mysql2' for database adapter, but the gem is not loaded. Add gem 'mysql2' to your Gemfile (and ensure its version is at the minimum required by ActiveRecord).

エラーメッセージに理由が書いてあります。読みましょう。