Wrong socket directory?

Sometimes there is a mismatch in actual socket directory and where the adapter thinks it is, then you'll see an error like this:

could not connect to server: No such file or directory

Is the server running locally and accepting
connections on Unix domain socket "/var/run/postgresql/.s.PGSQL.5432"?

In that case explicitely set the socket directory in database.yml like this:

development:
  adapter: postgresql
  host: /tmp
  ...