Async.fi

Note to self: Django/South basic usage

Not that I understand how this schema migration thing actually works. Anyway, initially, on an empty database: 
./manage.py syncdb --noinput
./manage.py schemamigration YOUR_APP --initial
./manage.py migrate YOUR_APP 0001 --fake
Then, after fiddling with your models: 
./manage.py schemamigration YOUR_APP --auto
./manage.py migrate YOUR_APP
That should do it. But like I said, my understanding of the system is very limited and I'm sure there are cases when this simplistic pattern just won't cut it. My needs, however, at the moment are not the most complicated.

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Note to self: Django's Error: cannot import name Foo

If Foo is definitely defined, then this is most likely a circular import which can be avoided by dropping the "redundant" import statement and referring not directly to the Model itself, but its' name. This is known as a lazy relationship: 
foo = models.ForeignKey('Foo')

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Reading EC2 tags with Boto

(Ouch! Looks like WordPress update to 3.1.3 wiped all the modifications I made to the default theme. Admittedly I should've seen that coming.) What I want to do is basically attach a key-value pair to an EC2 instance when launching it in AWS Management Console and read the value inside the instance when it's running. To be more specific, I use this to to set a key called environment that can have values like dev, stage and prod so that the Django config can decide which database to connect to etc. while starting up. I suspect that in Boto the current instance can somehow be referenced in a more direct fashion but this works as well. First, append the following to /etc/profile: 
# See: http://stackoverflow.com/questions/625644/find-out-the-instance-id-from-within-an-ec2-machine
export EC2_INSTANCE_ID="`wget -q -O - http://169.254.169.254/latest/meta-data/instance-id || die \"wget instance-id has failed: $?\"`"
test -n "$EC2_INSTANCE_ID" || die 'cannot obtain instance-id'
export EC2_AVAIL_ZONE="`wget -q -O - http://169.254.169.254/latest/meta-data/placement/availability-zone || die \"wget availability-zone has failed: $?\"`"
test -n "$EC2_AVAIL_ZONE" || die 'cannot obtain availability-zone'
export EC2_REGION="`echo \"$EC2_AVAIL_ZONE\" | sed -e 's:\\([0-9][0-9]*\\)[a-z]*\\$:\\\\1:'`"
Now we know the region and instance ID. Next, install Boto by running the following commands: 
wget "http://boto.googlecode.com/files/boto-2.0b4.tar.gz"
zcat boto-2.0b4.tar.gz | tar xfv -
cd boto-2.0b4
python ./setup.py install
Then, add these lines to ~/.profile: 
export AWS_ACCESS_KEY_ID=<ACCESS_KEY>
export AWS_SECRET_ACCESS_KEY=<SECRET_KEY>
Or the equivalent in ~/.boto: 
[Credentials]
aws_access_key_id = <ACCESS_KEY>
aws_secret_access_key = <SECRET_KEY>
Now, to read the tag we want in Python: 
#!/usr/bin/env python                                                                                                                                           

import os
from boto import ec2

ec2_instance_id = os.environ.get('EC2_INSTANCE_ID')
ec2_region = os.environ.get('EC2_REGION')

conn = ec2.connect_to_region(ec2_region)

reservations = conn.get_all_instances()
instances = [i for r in reservations for i in r.instances]

for instance in instances:
    if instance.__dict__['id'] == ec2_instance_id:
        print instance.__dict__['tags']['environment']

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