过滤MongoEngine中的嵌入列表(Filtering an embedded list in MongoEngine)

过滤MongoEngine中的嵌入列表(Filtering an embedded list in MongoEngine)

如果我有这些型号：

class Sub(EmbeddedDocument): name = StringField() class Main(Document): subs = ListField(EmbeddedDocumentField(Sub))

我想要一个返回Mains的查询，其中subs按名称进行过滤

Main.objects.filter(subs__name__exists=True)

这将返回正确的Mains，但Subs始终是整个列表，而不是子集。 我怎样才能获得子集？ 我需要依赖列表推导吗？

If I have these models:

class Sub(EmbeddedDocument): name = StringField() class Main(Document): subs = ListField(EmbeddedDocumentField(Sub))

I want to have a query that returns the Mains, with the subs being filtered by name existing

Main.objects.filter(subs__name__exists=True)

This returns the correct Mains, but the Subs are always the entire list, not a subset. How can I get only the subset? Do I need to rely on list comprehensions?

最满意答案

MongoDB不支持您正在请求的此操作，因此Mongoengine也不支持。

您可以对阵列（列表）执行切片操作，但不能对ad-hoc过滤执行切片操作。 在MongoDB数组中切片的工作方式类似于在Python中切片列表，您可以使用slice__关键字语法使用Mongoengine进行slice__ ：

Main.objects.filter(subs__name__exists=True).fields(slice__subs=[0,2])

这将返回从索引0开始的subs（即第一个元素），然后返回两个元素。

MongoDB doesn't support exactly this operation that you're requesting, and therefore neither does Mongoengine.

You can perform slicing operations on arrays (lists), but not ad-hoc filtering. Slicing in MongoDB arrays works similarly to slicing lists in Python, and you can do it with Mongoengine using the slice__ keyword syntax:

Main.objects.filter(subs__name__exists=True).fields(slice__subs=[0,2])

This will return the subs starting at index 0 (i.e. the first element) and returning two elements after that.