CODEWITHSUNDEEP
pythondjango
Claudio Shigueo Watanabe
Claudio Shigueo Watanabe

Reputation: 1003

Recommended way to do math on database to show in template in Django

I need to get some data from the database, do some calculation and show the results in the template.

From my research I saw that I could use django-mathfilters. But I read that it is not recommeded to include business logic in the template. So I researched for alternatives.

It seems that I could use annotate. So I have tried this:

def price_supplier(request):
    list = request.GET.get("list")
    price_list = SupplierPriceList.objects.filter(id=list).annotate(margin=(F('cod_product.sale_price') - F('price')) / F('cod_product.sale_price')).all()
    return render(request, 'painel_fornecedor/comparacao_precos.html', {'price_list': price_list})

I have tested this and got the error "Cannot resolve keyword 'cod_product.sale_price' into field." How is the recommended way to implement this?

Here is the model:

class Product(models.Model):
    cod_product= models.IntegerField(primary_key=True)
    descripction = models.CharField(max_length=120, blank=True, null=True)
    sale_price = models.DecimalField(max_digits=20, decimal_places=10, blank=True, null=True)

class SupplierPriceList(models.Model):
    cod_product = models.ForeignKey(Product, on_delete=models.PROTECT, blank=True, null=True)
    price = models.DecimalField(max_digits=20, decimal_places=10, blank=True, null=True)

Thanks in advance!

Upvotes: 0

Views: 90

Answers (1)

Bernardo Duarte
Bernardo Duarte

Reputation: 4264

To use a related field you need to use __ instead of . as a lookup.

def price_supplier(request):
    list = request.GET.get("list")
    price_list = SupplierPriceList.objects.filter(id=list).annotate(
        margin=(F('cod_product__sale_price') - F('price')) / F('cod_product__sale_price')
    ).all()
    return render(request, 'painel_fornecedor/comparacao_precos.html', {'price_list': price_list})

Upvotes: 1

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