rake db:migrate
报如下错误：
LoadError: Could not load 'active_record/connection_adapters/mysql2_adapter'. Make sure that the adapter in config/database.yml is valid. If you use an adapter other than 'mysql2', 'postgresql' or 'sqlite3' add the necessary adapter gem to the Gemfile. LoadError: libmysqlclient.so.18: cannot open shared object file: No such file or directory - /home/drjr/opt/ruby-2.4.1/lib/ruby/gems/2.4.0/gems/mysql2-0.4.5/lib/mysql2/mysql2.so
原因是mysql从5.6升级到5.7了，mysql2这个gem的版本是0.4.3，有点老了，不能支持mysql5.7。mysql2升级到0.4.9，就OK了。