Performing calculations on Django database values: views.py, template or Javascript?

Question

I have a Django project that interfaces with a PostgreSQL database; one of my tables is about 450 rows long and each row contains about a dozen columns.

Each column's value must be put into a different formula to produce the final data I want to display in the template layer, but each row will be processed with the same set of calculations. (In other words, I want to perform 12 different calculations 450 times each.)

My question is, of the following methods, which is the generally accepted method of doing so, and which is the best in terms of performance:

A) Write each calculation as a model method in models.py, query all objects of interest (again, about 450 of them) in views.py and pass them directly to the template, and then use Django's template language to write things like:

<table>
{% for item in list %}
    <tr>
         <td>{{item.name}}</td>
         ...
    </tr>
{% endfor %}
</table>

(where .name is a model method)

B) Peform the query and calculations in the views.py file and pass the results organized into one big dictionary or list or JSON to the template (and then parse it using template tags),

C) Pass all of the objects of interest to the template as in A), but perform calculations on each object's fields using Javascript or jQuery,

D) Combine B) and C) to perform all database queries in views.py and then pass the raw values as a big dictionary/list/JSON object to the template and perform calculations on those values using Javascript/jQuery.

To my mind, method A) seems the least efficient as it requires you to hit the database a ton of times in the template layer. But it makes writing the view and template extremely simple (just requires you to write 12 different model methods). Method B) seems to follow the general tenet of performing as much logic as possible in your view, and simply passing along the final results to be displayed in the template. But it makes the view function long and ugly. Methods C) and D) put most of load on the end user's browser, which seems like it could really take a load off the server, but then obviously won't work if the user has JS turned off.

But again, I'd like to know if there's an accepted best practice for this kind of situation, and whether that contradicts the fastest method from a computational standpoint.

And obviously, if I've missed the best method of all, please let me know what it would be.

rafaponieman · Accepted Answer · 2014-01-23 20:23:40Z

2

Methods (C) and (D), though tempting are not the way to go. The client's computer might be slow in which case the page might just freeze their browser while it performs all of these calculations.

(A) seems like the best option, and it shouldn't incur in additional queries. If you pass the objects to the template in a single queryset (Model.objects.all() for example), then Django should perform a single query to fetch all of the objects, then when calling the method in the template no additional query should be performed. If your objects are related to others, and this relation is used in the calculation, make sure to use select_related or prefetch_related because querying a related object on each model call will indeed incur in a single query per object unless you prefetch/select what you're going to use beforehand (https://docs.djangoproject.com/en/dev/ref/models/querysets/#select-related).

edited Jan 23, 2014 at 20:23

answered Jan 23, 2014 at 20:11

rafaponieman

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6 Comments

GChorn Over a year ago

Thanks for the response. I was under the impression that only (C) and (D) rely on the client's computer, while (B) relies on the server since all calculations are performed in the view layer and only final values are passed to the template?

rafaponieman Over a year ago

Yes, I had a typo, so it was C and D which rely on the client.

rafaponieman Over a year ago

So, in some very rare cases its necessary to build a python dictionary with your processed information and pass that to the view, as in (B), an example would be if you needed to make calculations that involve more than 1 register at a time, so you can't call a class method on each individual object, however, in general, its tidier to have a class method that does the calculations, use the view to fetch the necessary queryset, and then in the template just call that method and display the results however you prefer. You can use the {% with %} template tag to store the result of the method call.

GChorn Over a year ago

Thanks again @rafadev. For some reason I thought calling model methods queried the database; thanks for clearing that up. And thanks also for pointing me to .select_related(), though unfortunately it didn't speed up my application by much even though it cut the number of queries in half. I had been using method (A) but was looking for a way to improve my app's performance...I guess most of the slowdown is simply coming from the method calls, even though they're not hitting the db.

rafaponieman Over a year ago

Are you using Django debug toolbar? That will help you see each individual query, the time it took to query the DB and the time it took python to process all the information...

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