I’ve been looking into how to implement a tagging mechanism with Appengine (Python). By using a StringListProperty, you can associate a list of tags with an entity. The model would look something like this:

from google.appengine.ext import db

class Sample(db.Model):
  name = db.StringProperty(required=True)
  tags = db.StringListProperty()

Now, assuming we want to find the entities tagged with a given word, we can use a query like this:

q = db.GqlQuery(“SELECT * FROM Sample where tags = :1″,’z')

If we want to find all entities that have ANY of these tags (OR) we can query with GqlQuery or filter:

q = db.GqlQuery(“SELECT * FROM Sample where tags in :1″,['a','z'])

q = domain.Sample.all()
q.filter(“tags in “, ['b','d'])

When we want to find entities with ALL of the given tags:

q = db.GqlQuery(“SELECT * FROM Sample where tags = :1 and tags = :2″,’b',’c')
q = domain.Sample.all()
q.filter(“tags = “, ‘b’)
q.filter(“tags = “, ‘c’)

Too easy!

If you have an appengine project, you can try this code out by adding the entity (Sample – listed above) to your domain model and run the following in your development console (http://localhost:8080/_ah/admin/interactive):

import domain

domain.Sample(name='t1',tags=['a','b','c']).put()
domain.Sample(name='t2',tags=['b','c','d']).put()
domain.Sample(name='t3',tags=['c','d','e']).put()
domain.Sample(name='t3',tags=['x','Y','z']).put()

def display(r):
 for r in results:
  print r.name +" tags = "+' '.join(r.tags)

print "FIND WITH THIS TAG"
q = db.GqlQuery("SELECT * FROM Sample where tags = :1",'z')
results = q.fetch(5)
display(results)

print "FIND WITH ANY OF THESE TAGS (OR)"
q = db.GqlQuery("SELECT * FROM Sample where tags in :1",['a','z'])
results = q.fetch(5)
display(results)

print "FIND WITH ANY OF THESE TAGS (OR)"
q = domain.Sample.all()
q.filter("tags in ", ['a','z'])
results = q.fetch(5)
display(results)

print "FIND WITH ANY OF THESE TAGS (AND)"
q = db.GqlQuery("SELECT * FROM Sample where tags = :1 and tags = :2",'b','c')
results = q.fetch(5)
display(results)

print "FIND WITH ALL OF THESE TAGS (AND)"
q = domain.Sample.all()
q.filter("tags = ", 'b')
q.filter("tags = ", 'c')

results = q.fetch(5)
display(results)

This should produce the following result:

FIND WITH THIS TAG
t3 tags = x Y z
FIND WITH ANY OF THESE TAGS (OR)
t1 tags = a b c
t3 tags = x Y z
FIND WITH ANY OF THESE TAGS (OR)
t1 tags = a b c
t3 tags = x Y z
FIND WITH ANY OF THESE TAGS (AND)
t1 tags = a b c
t2 tags = b c d
FIND WITH ALL OF THESE TAGS (AND)
t1 tags = a b c
t2 tags = b c d

So, it’s easier than I thought. But I’m not experienced with DataStore yet, and I don’t know if there are inherit limitations with this approach – remember, there are limits on the way you retrieve data.

I’m getting back into appengine at the moment, and I came across this error:

No module named _multiprocessing

A google search turned up this error report:

http://code.google.com/p/googleappengine/issues/detail?id=1504

which lead to this post:

http://code.google.com/p/soc/wiki/GettingStarted

The problem was of-course, that I was using Python 2.6 instead of 2.5.

I love Linux, the solution is trivial:

The workaround for this is to install Python 2.5 on your machine. It will be then be accessible by using python2.5 on the command line. Then open thirdparty/google_appengine/dev_appserver.py in your favorite text editor and replace the first line,

#!/usr/bin/env python

with

#!/usr/bin/env python2.5

Note, I already had Python 2.5 installed via:

sudo apt-get install python2.5

Now, I’ll promptly get back to figuring out how best to use DataStore to implement my solution!

MyOnlineProfile.net has launched – as a Google AppEngine application.

MyOnlineProfile lets you catalog your public online profiles in one place – leaving you with just one link to give your friends and one link in your email signature.

I first wrote MyOnlineProfile as a Grails application – its a small application at the moment, so it didn’t take long. However I did over-engineer it. I made the classic mistake of trying to solve the wrong problems and over-optimising. Luckily with being a small application and written with such a productive framework, I’m only talking about wasting hours rather than months.

I deployed it to my Linux VPS, but unfortunately, Java based applications take up quite a bit of resources just starting up and memory on a VPS costs money (recurring money). So there just wasn’t room for it.

Hence re-writing it in Python for AppEngine. Being exposed to Python has been good for me – and relatively easy since Groovy opened my mind significantly and I was ready for the experience.

In addition to the language, the AppEngine environment is very much like Grails in that infrastructure is taken care of for you – just like database access is so transparent in Grails, it is in AppEngine.

These things make writing the application about the application rather than the technology or infrastructure – just the way I like it.

I am using prototype in a Google Appengine project, and while it worked on my development machine, after deploying it onto the Google infrastructure prototype wasn’t parsing the JSON responses anymore.

In my code, I was returning the JSON in the response text (as opposed to using X-JSON response header). The Content-Type response header was set as application/json and my javascript code used Ajax.Request() with the parameter evalJSON:true so as to parse the response text.

Running locally with the dev_appserver.py everything worked okay. However when running the deployed app, the Content-Type header was no longer being set – meaning that prototype would never parse the response text.

I’m not sure why Content-Type doesn’t get set when running on the Google infrastructure yet it works on the development server. But, luckily when using Ajax.Request, you can specify evalJSON:’force’ so that prototype parses the response regardless of the content-type.

Server ajax response code:

result = func(args)
# content type doesn't work when deployed
self.response.headers.add_header("Content-Type", 'application/json')
self.response.out.write(simplejson.dumps(result))

Javascript Ajax request:

new Ajax.Request('/rpc',{parameters:{action:'add'}
    ,method:'post',evalJSON:'force',onSuccess:function(transport) {
    var result=transport.responseJSON;
    ...
  }
}

Response headers from Google:

Content-Type:    text/html; charset=utf-8
Cache-Control:    no-cache
Content-Encoding:    gzip
Date:    Wed, 06 Aug 2008 09:56:05 GMT
Server:    Google Frontend
Content-Length:    82