Newbie: Django : Adding calculated results to Queryset before passing to template

Question

Hi All

Its day two of my new life with Django, please excuse the simplicity of my question.

I have an existing DB table(read-only access) that I have successfully displayed the contents of on a webpage using urls, views, models and all that good stuff.

The challenge I have is the table does not contain all the information I need to display. The table contains test results with the columns, sampletime, samplevalue, sampleresult. I need to display different data based on what I calculate from those columns.

My end goal is to display this info as a time series graph using flotr. For now Id be happy to just dump the data I need to a table on a web page.(So I can visualize the resulting data)

What Id like to pass to the template is,

• jssampletime(the sampletime datetime object converted to javascript epoch ms)
• resultvalue(rolling sum+- of samplevalue based on whether sampleresult was good or bad)

I'm fine with creating jssampletime and resultvalue using def functions. I presume I would add these functions to views.py

I guess what I need to do iterate over the a querySet in views.py and store the results in a list of dictionaries which I pass to the template. Something like this(code not tested).

views.py

# views.py
# Sudo code to assit in asking the question
from django.shortcuts import render_to_response
from thing.reporter.models import Samples

def _datetime_to_js(sampletime):
    #.. date conversion epoch magic
    return jsd_result

def _rolling_sum(samplevalue,sampleresult):
    #.. summing magic
    return sum_result

def dumptable(request): # The def that is called by urls.py
    object_list = Samples.objects.all()

    list_for_template = []
    for row in object_list:
        jssampletime = _datetime_to_js(row.sampletime)
        resultvalue  = _rolling_sum(row.samplevalue,row.sampleresult) 
        list_for_template.append({'jssampletime':jssampletime,'resultvalue':resultvalue})   

    return render_to_response('tabledump.html', {'result_list': list_for_template})

tabledump.html

# tabledump.html template
{% block content %}
    <h2>Results dumped to page for testing</h2>
    <ul>
    <table>
    {% for result in result_list %}
        <tr>
        <td>{{ result.jssampletime }}</td>
        <td>{{ result.resultvalue }}</td>
        </tr>
    {% endfor %}
    </table>
    </ul>
{% endblock %}

I think this would work but Im not sure if it is the Django MVC way.

Is it right that I,

• calculate the result I need in views.py by interating over a queryset result?
• pass my result to a template as a list of dict(is a queryset more than that)?

I guess Im looking for some direction and code tips. Am I on the right path ? Is there a better way ?

Answer 1

If the information you are displaying is in a model why not just add properties/methods to the model to display whatever information you need to get out of it? Then you can pass the actual model list / query set to your template and call the methods as properties.

e.g.

class MyModel(models.Model):
    model_field = models.CharField(max_length=255)

    @property
    def calculated_field(self):
        return self._do_calculation(self.model_field)

If you need access to state variables in your loop, don't forget that you can attach any property to a python object. This can be extremely useful. So in your view you could have something like:

for row in object_list:
    # update some variable we want to use in the template
    row.newly_added_field = run_calculation(row, somevariable)

Both of these could then be accessed within the template:

{% for result in result_list %}
    <tr>
    <!-- some stuff that displays the model directly -->
    <td>{{ result.calculated_field}}</td>
    <td>{{ result.newly_added_field}}</td>
    </tr>
{% endfor %}