没有关系时计数并返回零的注释 [英] Annotation to count and return zero when there is no relation

问题描述

给出以下关系：

class LicenseRequest:
    license_type = models.ForeignKey(LicenseType)
    created_at = models.DateField(default=now, editable=False)

class LicenseType:
    name = models.CharField(max_length=100)
    value = models.CharField(max_length=3, unique=True)

我想计算每个许可证已创建多少个请求类型。但是，由于我正在生成图形，因此在该特定时期内没有任何许可证请求的许可证类型必须包括0（零）。

I want to count how many requests have been created for each license type. However, since I am generating a graphic, I must include 0 (zero) for license types without any license request in that specific period.

我尝试执行建议的操作此处，但此操作无效。我只能从具有多个许可证请求的许可证类型中获得计数。

I tried to do what was suggested here, but it did not work. I can only get the count from License Types which have more than one license request.

qs = LicenseType.objects.filter(
                Q(licenserequest__created_at__range=(start_date, end_date)) | Q(licenserequest__isnull=True)
            ).annotate(rel_count=Count('licenserequest__id'))

我可以找到实现此目标的另一种方法，但我想知道是否可以通过注释来实现。

I could find another way to achieve this goal, but I was wondering if I can do it through annotation.

我正在使用django1.11.15 。

推荐答案

在 djang-2.0 及更高版本， Count 对象的 filter 参数，因此我们可以为此指定代码：

In djang-2.0 and higher, the Count object has a filter parameter, so we can specify the codntions for this:

qs = LicenseType.objects.annotate(
    rel_count=Count(
        'licenserequest',
        filter=Q(licenserequest__created_at__range=(start_date, end_date))
    )
)

对于 djang-1.11 及以下版本，我们可以使用案例（的 Sum（..） ..）表达式：

For djang-1.11 and below, we can use the Sum(..) of a Case(..) expression:

qs = LicenseType.objects.annotate(
    rel_count=Sum(Case(
        When(
            licenserequest__created_at__range=(start_date, end_date),
            then=1
        ),
        default=0,
        output_field=IntegerField()
    ))
)

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