setting source option after deploy does nothing

[Alex Chiang]

If i attempt to set the source to ppa:juju/stable before juju deploy, it doesn't seem to work, because juju doesn't know about the service. But if I set it after juju deploy, i don't see juju updating the package on the unit.

Some relevant bits of irc conversation

<abentley> achiang: It should not matter whether you set it at deploy time or not.
<abentley> achiang: Especially since it will take the instance a little while to start up, so you can set it before it is used.
<achiang> abentley: yeah, i was able to do that yesterday -- do a deploy and then immediately set the source. but that seemed like a weird way to use the option... "can i type faster than my metal will deploy?"

<abentley> achiang: A well-written charm will not care when you set the source, and will end up in the same state regardless. A poorly-written charm will error the unit if the source isn't set correctly at first.

The key here seems to be:

###
<abentley> achiang: Since the mongodb installation is affected by the 'source' setting, 'config-changed', not 'install' should install mongodb.
###

FWIW, this sequence resulted in success:

$ cat mongodb.yaml
mongodb:
    source: ppa:juju/stable

$ juju deploy --config mongodb.yaml cs:precise/mongodb

$ juju ssh mongodb/0

$ dpkg -l | grep mongo
ii mongodb 1:2.4.6-0ubuntu5~ubuntu12.04.1~juju1 object/document-oriented database (metapackage)
ii mongodb-clients 1:2.4.6-0ubuntu5~ubuntu12.04.1~juju1 object/document-oriented database (client apps)
ii mongodb-dev 1:2.4.6-0ubuntu5~ubuntu12.04.1~juju1 object/document-oriented database (development)
ii mongodb-server 1:2.4.6-0ubuntu5~ubuntu12.04.1~juju1 object/document-oriented database (server package)

$ apt-cache policy mongodb
mongodb:
  Installed: 1:2.4.6-0ubuntu5~ubuntu12.04.1~juju1
  Candidate: 1:2.4.6-0ubuntu5~ubuntu12.04.1~juju1
  Version table:
 *** 1:2.4.6-0ubuntu5~ubuntu12.04.1~juju1 0
        500 http://ppa.launchpad.net/juju/stable/ubuntu/ precise/main amd64 Packages
        100 /var/lib/dpkg/status
     1:2.4.6-0ubuntu5~ctools1 0
        400 http://ubuntu-cloud.archive.canonical.com/ubuntu/ precise-updates/cloud-tools/main amd64 Packages
     1:2.0.4-1ubuntu2.1 0
        500 http://nova.clouds.archive.ubuntu.com/ubuntu/ precise-updates/universe amd64 Packages
        500 http://security.ubuntu.com/ubuntu/ precise-security/universe amd64 Packages
     1:2.0.4-1ubuntu2 0
        500 http://nova.clouds.archive.ubuntu.com/ubuntu/ precise/universe amd64 Packages