Django-在模板上显示模型中字段的过滤后总和

Question

I have a model (camisaEvento) that have a One To Many relationship with (quantidadeEvento). I want to display on the template the sum of a field in quantidadeEvento but only for the elements of a determined record on camisaEvento.

On the view i have the following, which are passed to the template to a render_to_response:

camisasEvento = camisaEvento.objects.all()
quantidadesEvento = quantidadeEvento.objects.all()

I'm making a table for each record of camisaEvento, and in the end i want it to have a cell that is the sum of a determined field on the quantidadeEvento but only for the current camisaEvento record on the for loop, so in the template i have the following:

{% for camisa_do_evento in camisasEvento %}
<table>
    <td>...<td>
    <td>...<td>
    .
    .
    .
    <td>The sum should be in here</td>
<table>
{% endfor %}

What is the best way to do this? I know this is probably simple, but i'm quite new to programming and Django.

Thanks

Answer 1

You should use the aggregation framework to calculate the sum, in the view:

from django.db.models import Sum
camisasEvento = camisaEvento.objects.all().annotate(quantidade_sum=Sum('quantidadeevento__field_to_sum')

where 'field_to_sum' is the field you want to add up. This creates an attribute called quantidade_sum on each camisasEvento that contains the sum of the related fields.

Note that in your template, you can iterate through the quantidadeEvento objects related to each camisaEvento by using the reverse relationship:

<table>
    {% for camisa_do_evento in camisasEvento %}
    <tr>
        {% for quantidade_evento in camisa_do_evento.quantidadeEvento.all %}
          <td>{{ quantidade_evento.my_field }}<td>
        {% endfor }
        <td>Sum: {{ camisa_do_evento.quantidade_sum }}</td>
    </tr>
    {% endfor %}
<table>

Answer 2

Why not using a property on camisaEvento?

class camisaEvento(models.Model):

    ...

    @property
    def sum_quantity(self):
        # just guessing, don't know exactly what you need
        # it's using python's list comprehension, but you can use a more explicit
        # syntax if you're not familiar with this one
        return sum([obj.field_to_sum for obj in               
                    quantidadeEvento.objects.filter(fk_field=self.id)])

and then, in your template:

{% for camisa_do_evento in camisasEvento %}
    {{ camisa_do_evento.sum_quantity }}
{% endfor %}

How does this work: I'm creating a method on the camisaEvento class that fetches all the quantidadeEvento for that class and sums up their field (that's "fantasy" logic, since I don't exactly know what you need to sum). Then, in your template, you just call that method as if it was a real property of your camisaEvento class