Django:注释不起作用? [英] Django: Annotation not working?

问题描述

我有两个模型，产品，与RatingEntry具有一对多关系:

I have two models, Product, which has a one-to-many relationship with a RatingEntry:

>>> product_entries = models.Products.objects.all()
>>> annotated = product_entries.annotate(Count("ratingentry"))
>>> len(annotated)
210
>>> a = annotated.filter(ratingentry__count__lte = 10)
>>> b = annotated.filter(ratingentry__count__gt = 10)
>>> len(a)
10
>>> len(b)
200
>>> len(a | b)
10 //should be 210

如果我将a和b更改为列表，然后将它们串联起来，则长度为210.

If I change a and b to lists, and concatenate them, the length works out to 210.

知道这是怎么回事吗?

推荐答案

我认为此行为是Django对象关系映射中的错误.如果您查看Django为查询生成的SQL，那么您将看到类似以下的内容:

I think this behaviour is a bug in Django's object-relationship mapping. If you look at the SQL that Django generates for your query, then you'll see something like this:

>>> q1 = (Products.objects.annotate(num_ratings = Count('ratingentries'))
...       .filter(num_ratings__gt = 10))
>>> q2 = (Products.objects.annotate(num_ratings = Count('ratingentries'))
...       .exclude(num_ratings__gt = 10))
>>> print(str((q1 | q2).query))
SELECT `myapp_products`.`id`, COUNT(`myapp_ratingentries`.`id`) AS
`num_ratings` FROM `myapp_products` LEFT OUTER JOIN `myapp_ratingentries` ON
(`myapp_products`.`id` = `myapp_ratingentries`.`product_id`) GROUP BY
`myapp_products`.`id` HAVING COUNT(`myapp_ratingentries`.`id`) > 10
ORDER BY NULL

请注意，来自q1的条件包含在查询的HAVING子句中，但是来自q2的条件已丢失.

Note that the condition from q1 is included in the HAVING clause of the query, but the condition from q2 has been lost.

您可以通过以下方式构建查询来解决此问题:

You can work around the problem by building your query like this:

>>> q = Q(num_products__gt = 10) | ~Q(num_products__gt = 10)
>>> q3 = Products.objects.annotate(num_ratings = Count('ratingentries')).filter(q)
>>> print(str(q3.query))
SELECT `myapp_products`.`id`, COUNT(`myapp_ratingentries`.`id`) AS
`num_ratings` FROM `myapp_products` LEFT OUTER JOIN `myapp_ratingentries` ON
(`myapp_products`.`id` = `myapp_ratingentries`.`product_id`) GROUP BY
`myapp_products`.`id` HAVING (COUNT(`myapp_ratingentries`.`id`) > 10 OR NOT
(COUNT(`myapp_ratingentries`.`id`) > 10 )) ORDER BY NULL

请注意，这两个条件现在都包含在HAVING子句中.

Note that both conditions are now included in the HAVING clause.

我建议您将此错误报告给Django开发人员. (如果无法解决，则至少应记录在案.)

I suggest that you report this to the Django developers as a bug. (If it can't be fixed, then at least it should be documented.)

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