Django views: how to modify returned queryset before rendering in template?

Question

I'm trying to make a simple weather JSON API with Python 2.7 and Django 1.5.

My WeatherData model looks like this:

class WeatherData(models.Model):
    created_at = models.DateTimeField(auto_now_add=True)
    temp_f = models.DecimalField()

In my views, I want to convert the created_at datetime object stored by Django into UTC seconds. (I understand there will be a timezone problem.)

I already know how to convert a datetime object (named MYTIME) to seconds:

import time
time.mktime(MYTIME.timetuple())

However, when I define the queryset = WeatherData.objects.all() I can't figure out a way to convert these datetime objects on-the-fly to UTC seconds while maintaining the queryset object. In short, I want to modify a returned queryset in the views before rendering in template.

I'm new to using Django and MySQL but I imagine there's a way to do this.

Note: I'm using TastyPie, so I don't have direct access to a template file. I specify the queryset fields in a ModelResource class:

class WeatherResource(ModelResource):
    class Meta:
        queryset = WeatherData.objects.all()
        fields = ['created_at' 'temp_f']

Thanks for the help in advance!

Are you trying to modify the data solely for the purpose of rendering it in your template, or is it so you can then perform other operations (i.e. sorting, choosing a template dynamically, etc.)? If it's the former, you won't want to modify the queryset at all ... rather you'd use a template tag that would do the conversion at the time of rendering. If it's the latter, then there are several approaches you could try. Provide a bit more of the context to get some specific solutions. — jlmcdonald
It's just for rendering. Specifically, I need to serialize the datetime object into JSON. But since I'm using TastyPie, I don't have access to the template file. — hao_maike

Paulo Bu Paulo Bu · Accepted Answer · 2013-05-16T18:51:47

You don't have to modify it, just add a method to the WeatherData class that do this and call the method within the template for each object in the query set:

import time
class WeatherData(models.Model):
    created_at = models.DateTimeField(auto_now_add=True)
    temp_f = models.DecimalField()

    def created_at_utc(self):
        # whatever logic you need to do the conversion
        return time.mktime(self.created_at.timetuple())

and within your template do this:

{% for data in deatherdata %}
    data.created_at_utc
{% endfor %}

I hope it helps!

Django views: how to modify returned queryset before rendering in template?

1 Answers