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If you look in your config/database.yml file, you are probably specifying localhost as the database host name, and you are probably not supplying a password.
Using localhost automatically switches the connection to a TCP connection, and by default in postgres, TCP connection auth method is md5 (password required).
Here are two ways to solve:
Switch to UNIX Socket connection - If you’re just running the postgres db in the same VM as Rails, you can switch from TCP to UNIX Socket connection by removing localhost from the host parameter in config/database.yml:
from:
development:
adapter: postgresql
...
host: localhost
...
to:
development:
adapter: postgresql
...
host: ''
...
Specify a Password: - If you need to connect over TCP, make sure the username you are using in config/database.yml is configured in postgres and set a password by connecting to the postgres db from the command line and running:
ALTER USER postgres PASSWORD 'myPassword';
and then specify the same password in your config/database.yml file:
development:
adapter: postgresql
...
host: localhost
password: 'myPassword'
...