Django查询不同币种钱包的总余额

Question

获取不同币种的用户钱包总余额的最佳方式是什么？

myapp/models.py

from django.db import models
from django.contrib.auth.models import User

USD = 'USD'
EUR = 'EUR'
GBP = 'GBP'
CURRENCY_CHOICES = (
    (USD, 'US Dollars'),
    (EUR, 'Euro'),
    (GBP, 'UK Pounds'),
)

class Wallet(models.Model):
    user = models.ForeignKey(User)
    accnumber = models.CharField(max_length=12)
    currency = models.CharField(choices=CURRENCY_CHOICES, default=EUR)
    balance = models.DecimalField(max_digits=9,decimal_places=2)

汇率从 fixer.io 获得以字典形式 [(u'usd', '美元'), (u'eur', '欧元'), (u'rub', '俄罗斯卢布'), (u'gbp', '英国英镑' ), (u'btc', '比特币'), (u'eth', '以太坊')]

myapp/views.py

import requests
from decimal import Decimal
from django.conf import settings
from django.views.generic.base import TemplateView
from django.db.models import Sum
from myyapp.model import Wallet, CURRENCY_CHOICES 

class TotalBalanceView(TemplateView):
    template_name = 'balance.html'

    def get_context_data(self, **kwargs):
        context = super(TotalBalanceView, self).get_context_data(**kwargs)

        #get current exchage rates from Fixer.IO
        symbols = ','.join(dict(CURRENCY_CHOICES).keys()).upper()
        uri = "http://data.fixer.io/api/latest?access_key={}&base=EUR&symbols={}".format(FIXER_API, symbols)
        r = requests.get(uri)
        rates = r.json()['rates']

        #get account for the user
        wallets = Wallet.objects.filter(user=self.request.user)
        total = Decimal()
        for curr in CURRENCY_CHOICES:
            total += wallets.filter(currency=curr).aggregate(
                    total=Sum('balance'))

        context.update({
            'wallets ': wallets
            'total': total_eur + total_usd * rates['USD'] + total_gbp * rates['GBP']
        })

        return context

myapp/templates/balance.html

<h1>Total is: {{ total|floatformat:2 }}</h1>
{% for w in wallets %}
    <p>{{ w.accnumber }}</p>
{% endfor %}

我确信应该有更有效的解决方案在一个查询请求中使用聚合函数

Answer 1

我们基本上可以通过按货币执行某种分组来做到这一点:

totals = (wallets.values('currency')
                 .annotate(total=Sum('balance'))
                 .order_by('currency'))

这将产生字典的可迭代，其中currency映射到货币，以及映射的total到该货币的总额。例如:

[{'currency': 'EUR', 'total': 123.45},
 {'currency': 'USD', 'total': 456.78},
 {'currency': 'BFR', 'total': 901.23}]

然后我们可以计算一种货币的总计:

total = sum([subsum[total] * rate[subsum['currency']]
             for subsum in totals])

您可能需要为要转换的货币添加汇率(汇率等于1.00)。