Django: Double underscore

最新推荐文章于 2024-12-29 18:50:49 发布

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博客介绍了Django在查询中跨关系的强大且直观的方法，它能自动处理SQL JOINs。可通过双下划线分隔的相关字段名跨模型查询，深度不限，也支持反向关系查询。若中间模型无符合条件的值，Django不会报错。

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Lookups that span relationships

Django offers a powerful and intuitive way to “follow” relationships in lookups, taking care of the SQL JOINs for you automatically, behind the scenes. To span a relationship, just use the field name of related fields across models, separated by double underscores, until you get to the field you want.

This example retrieves all Entry objects with a Blog whose name is ‘Beatles Blog’:

Entry.objects.filter(blog__name=‘Beatles Blog’)

This spanning can be as deep as you’d like.

It works backwards, too. To refer to a “reverse” relationship, just use the lowercase name of the model.

This example retrieves all Blog objects which have at least one Entry whose headline contains ‘Lennon’:

Blog.objects.filter(entry__headline__contains=‘Lennon’)

If you are filtering across multiple relationships and one of the intermediate models doesn’t have a value that meets the filter condition, Django will treat it as if there is an empty (all values are NULL), but valid, object there. All this means is that no error will be raised. For example, in this filter:

Blog.objects.filter(entry__authors__name=‘Lennon’)

(if there was a related Author model), if there was no author associated with an entry, it would be treated as if there was also no name attached, rather than raising an error because of the missing author. Usually this is exactly what you want to have happen.