mysql - 注释 Django 1.8 中复杂子查询的结果到 QuerySet

Question

我必须遵循数据库架构:

main_account
- username

invoice
- amount
- currency: 'EUR' or 'USD' or 'GBP'
- username (main_account has many invoices)

and my goal is to list the "top payers" in dollars. Thus, I have to calculate a "total_paid" sum for each user from the invoice table (and take currency conversions into account). I also wish to filter by this "total_paid".

My current SQL looks like:

SELECT main_account.username, total_paid
FROM main_account JOIN
(
    SELECT username,
        (SELECT SUM(amount) FROM invoice WHERE main_account.username = invoice.username AND invoice.currency = 'EUR') AS total_eur,
        (SELECT SUM(amount) FROM invoice WHERE main_account.username = invoice.username AND invoice.currency = 'USD') AS total_usd,
        (SELECT SUM(amount) FROM invoice WHERE main_account.username = invoice.username AND invoice.currency = 'GBP') AS total_gbp,
        (SELECT (IFNULL(total_usd,0) + 1.12 * IFNULL(total_eur,0) + 1.41 * IFNULL(total_gbp,0))) AS total_paid
    FROM main_account
) as tbl
ON main_account.username = tbl.username
WHERE total_paid >= 2000
ORDER BY total_paid

我想知道如何使用 Django 的 ORM 完成此任务。

看来解决方案是这样的:

MainAccount.objects
    .annotate(total_paid=???)
    .filter(total_paid__gte=2000)
    .order_by('total_paid')

一些注意事项:

• MainAccount.extra(...).filter(...).order_by(...) 将不起作用。无法过滤在 extra 中创建的值。

• 我已经尝试过 MainAccount.annotate(total_paid=RawSQL(...))，它运行良好，但在添加 .filter() 时遇到了一个奇怪的错误。出于某种原因，过滤器调用使用 SQL 参数改变 RawSQL 对象，然后抛出 "not all arguments converted during string formatting" 错误。

Answer 1

正如 BogdiG 所指出的，Django 1.8 条件表达式是解决方案。

Python/Django:

MainAccount.objects.annotate(
    total_paid = Sum(
        Case(
            When(invoices__currency='EUR', then=F('invoices__amount') * 1.12),
            When(invoices__currency='USD', then=F('invoices__amount')),
            When(invoices__currency='GBP', then=F('invoices__amount') * 1.41)
        )
    )
).filter(total_paid__gte=2000).order_by('total_paid')

生成的 SQL 类似于:

SELECT  main_account.username, 
        SUM(
            CASE 
                WHEN invoice.currency = 'EUR' THEN (invoice.amount * 1.12) 
                WHEN invoice.currency = 'USD' THEN invoice.amount 
                WHEN invoice.currency = 'GBP' THEN (invoice.amount * 1.41) 
                ELSE NULL 
            END
        ) AS total_paid 
FROM main_account 
    INNER JOIN invoice ON ( main_account.username = invoice.username ) 
GROUP BY main_account.username 
HAVING total_paid >= '2000' 
ORDER BY total_paid;