Quick Django Trick

Recently while playing around with a Django model in the always awesome iPython shell I discoved a neat feature of the Django ORM.  It's basically a way to get the id of a related object without actually triggering a query to get all of the related object's data.

Frequently when working with a model which has a foreign key, I simply want to access the id of the related item and I don't care about any of the related item's other information.  Situations where this comes up include generating links and building queryset filters.  Unfortunately if I follow the normal Django style and do something like "item.related.id", the Django ORM will fire off an extra query to get all of the information of the related object (which I don't care about).  While this is far from tragic it is still unnecessary work since all I care about is the object's id and that is already contained in the "item" object.

Fortunately there is an alternative!  Instead of getting the id via "item.related.id" one can say "item.related_id".  Using this method, no extra query is performed and I get just the id value I was looking for.

There are two things to be aware of with this trick.  First I have not found this feature documented anywhere with a cursory search of the Django docs.  This means that I am not sure how much this feature is actually supported and how permament it may or may not be.  Second while I have not tested it, I suspect that if your foreign key has a custom database field name, the field on the model will match that custom field name.

As I come across any other quick Django tricks I may start making them a regular feature here on the blog.  If you found this intersting or have your own quick django trick, let me know and leave a comment!

Update to django-ae-utils

I've been hanging out at the NYC App Engine Hackathon today and I've had some time to work on django-ae-utils.  Accordingly I've just posted an update to django-ae-utils.  This update includes a User model which uses the App Engine datastore as a backend, as well as a few generic views for working with the model.

Check it out at Google Code!

Introducing django-ae-utils

Recently I've started to play around with using Django on Google's new App Engine.  As mentioned by many others at this point, Django technically runs on App Engine but there are significant parts which do not.  Accordingly I've started to write some code to replace or supplement the holes left in Django by App Engine.  Since I figure this code may be of use to others I've released it as an open-source project called django-ae-utils (github mirror coming soon).

Installing Django on OS X 10.5 (Leopard)

This is a quick update to my previous installing django posts.

Today I got my hands on Leopard, so far I've been happy but went I went about reinstalling Django I hit a snag.  MacPort would encounter an error whenever it tried to build PostgreSQL.  If you can wait a little while the port maintainer for PostgreSQL should fix the problem.

On the other hand if you need Django running today you can follow the steps at Lee Packham's Corner to get PostgreSQL running.  Unfortunately this will get you PostgreSQL 8.2 (not inherently a bad thing) which means you will need to manually build and install psycopg2 since the MacPorts psycopg2 package depends on PostgreSQL 8.1.  Here's what you will need to do to manually build and install pyscopg2:

  • Download the source from http://www.initd.org/tracker/psycopg/wiki/PsycopgTwo.

  • Extract the files using the command tar -xzf psycopg2-latest.tar.gz.

  • Open the file setup.cfg file and look for the line that starts with pg_config=

  • Change the line to read pg_config=/opt/local/var/macports/software/postgresql82/8.2.5_0+darwin_9/opt/local/lib/postgresql82/bin/pg_config

  • Run the command python setup.py build

  • Run the command python setup.py install

Now you should be all set!

On a related note if your interested in learning about Djago and you'll be around Poughkeepsie, NY this Wednesday stop by at the MHVLUG meeting, I'll be giving a Intro to Django talk.

Installing and Running Django on a Mac - Part 2 - Install

This is a continuation of Part 1 - Getting Ready.

Let's just get into it.  Before you can run Django you will need to install several dependencies.  To do this I'm using MacPorts but it can be done just as easily using PortAuthority.  In PortAuthority just search for the package name, select the package in the results and click on the install button (the brown box thing).  With MacPorts open a terminal and run the command sudo port package_name.  Be aware that each of these will take a while to run while MacPorts downloads and compiles each package and its dependencies.

  • python25 - Python 2.5 (Django will run on any version of Python 2.3 or newer, I personally like 2.5)

  • openssl - Handles encryption between python and the database server.

  • py25-setuptools - Setup tools for Python 2.5

  • postgresql81 postgresql81-server - My database of choice.  Obviously install the database that makes sense for you.

  • py-psycopg2 - Python interface for postgres.  Additionally py-psycopg2 will install two dependencies, PortAuthority/MacPorts will take care of

  • subversion - Django doesn't need SVN to run but SVN is the easiest way to get the latest version of Django.

Installing and Running Django on a Mac - Part 1 - Getting Ready

I'm a recent Mac convert.  For the most part I've loved all the perks of being a Mac user but I've run into a road block recently.  Apparently getting my favorite web application framework, Django and its various dependencies running on OS X to do local development is not as easy or clear as it is on other platforms like windows or linux.  This may be thanks to my limited knowledge of BSD's querks or simply that not many people run Django on the Mac.  To make things even more complicated I want to run the NewFormsAdmin branch instead of the current 0.96 release.  Since I'm sure I'm not the only person running into troubles and confusion with this here's a step-by-step guide on how to get Django running on your Mac!

  • Install Xcode - You will need the tools included in Xcode to build and install many of Django's Pre-requisites.  To get Xcode you can either install it from your OS X install DVD or you can download it from Apple Developer Connection (ADC).  If you choose to download Xcode be aware that you will need to register (for free) and that the download is a little under 1 GB in size.  Otherwise if you install from DVD remember to run Software Update (System Prefrences > System > Software Update), there have been several updates to Xcode since its release.

  • Download and install MacPorts - What's MacPorts you ask?  MacPorts is a package mananagement tool that handles obtaining, building, and installing dozens of open source applications and tools onto a Mac.  Once you download MacPorts mount the DMG image and run the installer package.  Generally accepting the defaults for the installer is fine.  Once installation you will have a new command available in your terminal called port.

  • (Optional) Download and install PortAuthority - PortAuthority is a GUI frontend to MacPorts.  I love the command-line but sometimes for things like package managers a GUI can be nice to more easily search and browse around.  NOTE - PortAuthority costs $20 but offers a 30 day trial.

At this point you have all the resources you will need to install Django and it's dependencies.   Check back tomorrow to see how to do the actual install.