I have this configuration:
development: adapter: mysql2 encoding: utf8 database: my_db_name username: root password: my_password host: mysql://12
You should separate the host from the port number. You could have something, like:
development: adapter: mysql2 encoding: utf8 database: my_db_name username: root password: my_password host: 127.0.0.1 port: 3306