Database.yml.j2

default: &default
  adapter: postgresql
  encoding: unicode
  pool: 5
  timeout: 5000

development:
  <<: *default
  host: {{ host }}
  database: {{ database }}
  username: {{ username }}
  password: {{ password }}

------------------------------------------------

actions.py

def handle_pg(service_name):
  pg = extra_services.PostgresRelation()

  helpers.render_template(
    host=pg[pg.name][0]["host"],
    user=pg[pg.name][0]["user"],
    password=pg[pg.name][0]["password"],
    database=pg[pg.name][0]["database"],
    source='database.yml.j2',
    target='/var/database.yml')

-------------------------------------------------

services.py

#!/usr/bin/python

from charmhelpers.core.services.base import ServiceManager
from charmhelpers.core.services import helpers

import actions
import extra_services

def manage():
  manager = ServiceManager([
    {
      'service': 'rails-charm',
      'ports': [80, 443],  # ports to after start
      'provided_data': [
        # context managers for provided relations
        # e.g.: helpers.HttpRelation()
      ],
      'required_data': [
        extra_services.PostgresRelation()
        # data (contexts) required to start the service
        # e.g.: helpers.RequiredConfig('domain', 'auth_key'),
        #       helpers.MysqlRelation(),
      ],
      'data_ready': [
        helpers.render_template(
          source='upstart.conf',
          target='/etc/init/rails-charm'),
        actions.log_start,
        actions.handle_pg,
      ],
    },
  ])
  manager.manage()

-------------------------------------------------

stack trace

2015-01-15 00:10:40 INFO db-relation-changed Traceback (most recent call last):
2015-01-15 00:10:40 INFO db-relation-changed   File "/var/lib/juju/agents/unit-rails-charm-0/charm/hooks/db-relation-changed", line 3, in <module>
2015-01-15 00:10:40 INFO db-relation-changed     services.manage()
2015-01-15 00:10:40 INFO db-relation-changed   File "/var/lib/juju/agents/unit-rails-charm-0/charm/hooks/services.py", line 33, in manage
2015-01-15 00:10:40 INFO db-relation-changed     manager.manage()
2015-01-15 00:10:40 INFO db-relation-changed   File "/usr/local/lib/python2.7/dist-packages/charmhelpers/core/services/base.py", line 120, in manage
2015-01-15 00:10:40 INFO db-relation-changed     self.reconfigure_services()
2015-01-15 00:10:40 INFO db-relation-changed   File "/usr/local/lib/python2.7/dist-packages/charmhelpers/core/services/base.py", line 151, in reconfigure_services
2015-01-15 00:10:40 INFO db-relation-changed     self.fire_event('data_ready', service_name)
2015-01-15 00:10:40 INFO db-relation-changed   File "/usr/local/lib/python2.7/dist-packages/charmhelpers/core/services/base.py", line 198, in fire_event
2015-01-15 00:10:40 INFO db-relation-changed     callback(service_name)
2015-01-15 00:10:40 INFO db-relation-changed   File "/var/lib/juju/agents/unit-rails-charm-0/charm/hooks/actions.py", line 17, in handle_pg
2015-01-15 00:10:40 INFO db-relation-changed     target='/var/database.yml')
2015-01-15 00:10:40 INFO db-relation-changed TypeError: __init__() got an unexpected keyword argument 'database'
2015-01-15 00:10:40 ERROR juju.worker.uniter uniter.go:486 hook failed: exit status 1