Here is a simple Ruby code to connect to PostgreSQL.

gem 'pg'
require 'pg'
 
conn = PG::Connection.open(:dbname => 'test')
res  = conn.exec('SELECT * from table1')
res.each do |row|
  row.each do |column|
    puts column
  end
end
p conn
conn.close

If you look deep in PG::Connection library, it contains 7 parameters which you can use just some of them or all of them. Then you can execute your query by the following command.

Note: Your PostgreSQL should have a role set to your database.

CONNECT_ARGUMENT_ORDER = %w[host port options tty dbname user password]