Mein Frage drehte sich um Zeile 15, aber seitdem ich das *rescue nil* da stehen habe, klappt der Verbindungsaufbau :-)

setup.rb

require 'pg'
require 'active_record'

connection_details = {
  adapter: 'postgresql',
  database: 'toner',
  username: 'sebastian',
  encoding: 'utf8',
  pool: 5
}

# setup the database connection
ActiveRecord::Base.establish_connection(connection_details.merge({databse: 'postgres', schema_search_path: 'public'}))
# create a new toner database
ActiveRecord::Base.connection.create_database(connection_details.fetch(:database)) rescue nil
# establis a database connection
ActiveRecord::Base.establish_connection(connection_details)
# migrate ALL THE THINGS
ActiveRecord::Migrator.migrate("db/migrate/")