For the last week or so I've been wrapping my brain around Solr and Solango. The whole time that I've been doing this I've had the feeling that they can do awesome, powerful things but they're documentation is so poor that I couldn't figure anything out beyond the basic examples. Ultimately I had to dig through a bunch of code and do some experimentation. Now that I've finally figured out how to do what I've been trying to do and have wrapped my brain around some of the trickier bits I'm going to share some of the gotchas and solutions I've found.
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!
pictures, or a box with random comments the user has made.
Ideally in this type of situation I would simply preform a query with what ever rules I need and a random "order by" statement such as the following:
pictures = User.picture_set.filter(private=False).order_by('?')[:6]
Which generates SQL similar to:
SELECT * FROM `pictures`
WHERE private = 0 AND user_id = 5
ORDER BY rand()
In an ideal world this works great, I only get back as much data as I need and the site stays interesting because what the user sees is constantly changing/rotating. Unfortunately unless you are using PostgreSQL or have less than a few hundred rows in the pictures table you are not in an ideal world. Under both MySQL 4 and 5 random ordering is horrifyingly inefficient. Adding that "ORDER BY rand()" can make a query that took a few milliseconds to run instead take tens of seconds. The problem gets even worse when you are working with a table containing millions of rows and thousands of rows which meet the criteria of your query. What is one to do?
If your interested in getting an intro to Django and meeting up with at least one person (possibly more) who develop web apps with Django, stop by the MHVLUG meeting next Wednesday, November 7 at 6 PM in the MHVLS Auditorium! Check back soon for copies of all my presentation materials.
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.
- 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.