14 Feb 09 Member Map Part 2

I’ve gotten the Member Map application ported to my Django powered site now. This was pretty straightforward and a lot of fun because of the Javascript aspect. Let me address the points I made in part 1 of this post, below.

1. Leveraging jQuery. I did make use of jQuery for the client side Javascript. This really did simplify things. In the old version of the code, I had a 282 line Javascript file. The new version weighs in at 161 lines, which is 57% the size of the earlier version.
2. Use of JSON encoder library. It turns out that Django includes a version of the simplejson library already. This is used in Django’s serialization feature. I probably could have used this feature, but I didn’t need to serialize an entire model, and wanted to rename some of the fields. So I simply did a “import django.utils.simplejson as json” and went to town. My server side code was now much simpler than the previous version where I encoded by hand.
3. I did continue to use the markItUp! editor to let users edit their message to go along with their location. I learned that the size of the editor is easily adjusted through the use of CSS. So I was able to override the default size of the editor by including my own CSS style sheet below the default sheet.
4. Reducing complexity. Here is where it got interesting. I decided to add a “json” field to the MapEntry model. This field is populated whenever a model object is saved by providing my own save() method. Inside my save() I use Django’s template render_to_string() to render the HTML  message that appears in the pop-up balloon on the map. This consists  of the user’s avatar followed by a message that is marked up in Markdown. I then build a JSON representation of the MapEntry using Django’s simplejson. This representation consists of the following fields: name, location, latitude, longitude, and the HTML message. By pre-saving all of this information, it makes it much more efficient to retrieve 200+ users’ information when the map is first loaded. In the view function, I make use of the values_list() queryset method to retrieve all the json fields. I don’t need the ORM to convert each MapEntry to an actual Python object.

Another thing I did differently for the port was when a user updates or adds their position on the map, I don’t reload the entire map. I just adjust their information to reduce bandwidth with an AJAX post. I’m not sure why I didn’t do this before, perhaps because it was to tedious to do without jQuery. It was just easier to reload the whole thing.

This was all well and cool, doing all this pre-computation, but what happens when a user changes her avatar? Her entry on the map will likely have a broken graphic. No problem. Here I took advantage of Django’s signals. I attached a signal handler to listen for changes to the UserProfile model. Whenever the UserProfile is saved, my signal handler runs and re-saves the corresponding MapEntry to regenerate the JSON. Very slick.

And finally, another thing that I learned about Django was that simple_tags can have defaulted arguments. I added an argument to my avatar tag so that I could apply CSS to the generated HTML img tag if needed. In the Member Map pop-up balloon, it looked nicer if the avatar was floated to the left.

So all and all, again, I’m very happy with how easy Django makes writing an application like this. Using jQuery was also a big productivity booster.

Here are some screenshots. You can also see the new Blueprints CSS in action as well.

Member Map 1

Member Map 2

Tags: django, jquery, json, markitup, membermap