查询针对单个文档集合运行。 （当前）无法跨多个（子）集合进行查询。

A query runs against a single document collection. There is (currently) no way to query across multiple (sub)collections.

这意味着您可能希望将用户文档保留在顶层 documents 集合，其UID具有所有者属性。然后，您可以选择带有简单 documentsRef.where（ owner， ==，uid）的用户帖子。

That means you'll probably want to keep the user documents in a top-level documents collection, with a owner property for their UIDs. Then you can select a user's post with a simple documentsRef.where("owner", "==", uid).