When using the host: configuration option in the database.yml set to localhost or 127.0.0.1, I would need to add an entry to PostgreSQL’s pg_hba.conf file to allow my ip address access. But, if you give the host: option the directory of your PostgreSQL sockets, rails will be able to use the socket, without needing to add an entry to the PostgreSQL configuration file.
production:
database: fancy_things
host: '/var/run/postgres'
...