Django 条件注释

发布于 2024-11-29 05:59:11 字数 283 浏览 0 评论 0原文

我很惊讶这个问题显然还不存在。如果有的话，请帮我找到它。

我想使用 annotate (Count) 和 order_by，但我不想计算相关对象的每个实例，只计算那些满足特定条件的实例。

也就是说，我可以根据燕子携带的绿色椰子的数量来列出它们：

swallow.objects.annotate(num_coconuts=Count('coconuts_carried__husk__color = "green"').order_by('num_coconuts')

原文

I'm surprised that this question apparently doesn't yet exist. If it does, please help me find it.

I want to use annotate (Count) and order_by, but I don't want to count every instance of a related object, only those that meet a certain criteron.

To wit, that I might list swallows by the number of green coconuts they have carried:

swallow.objects.annotate(num_coconuts=Count('coconuts_carried__husk__color = "green"').order_by('num_coconuts')

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水晶透心 2024-12-06 05:59:11

对于 Django >= 1.8：

from django.db.models import Sum, Case, When, IntegerField

swallow.objects.annotate(
    num_coconuts=Sum(Case(
        When(coconuts_carried__husk__color="green", then=1),
        output_field=IntegerField(),
    ))
).order_by('num_coconuts')

For Django >= 1.8:

from django.db.models import Sum, Case, When, IntegerField

swallow.objects.annotate(
    num_coconuts=Sum(Case(
        When(coconuts_carried__husk__color="green", then=1),
        output_field=IntegerField(),
    ))
).order_by('num_coconuts')

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木槿暧夏七纪年 2024-12-06 05:59:11

这应该是正确的方法。

swallow.objects.filter(
    coconuts_carried__husk__color="green"
).annotate(
    num_coconuts=Count('coconuts_carried')
).order_by('num_coconuts')

请注意，当您过滤相关字段时，在原始 SQL 中，它会转换为 LEFT JOIN 加 WHERE。最后，注释将作用于结果集，该结果集仅包含从第一个过滤器中选择的相关行。

This should be the right way.

swallow.objects.filter(
    coconuts_carried__husk__color="green"
).annotate(
    num_coconuts=Count('coconuts_carried')
).order_by('num_coconuts')

Note that when you filter for a related field, in raw SQL it translates as a LEFT JOIN plus a WHERE. In the end the annotation will act on the result set, which contains only the related rows which are selected from the first filter.

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