为什么要先调用toList，然后再调用flatten?如果我正确地阅读了BSONDocument的文档，则getAs方法将为类型T返回Option，在这种情况下，您将T指定为List[BSONObjectID].调用toList时，基本上是将Option拆包为空的List(对于None)或对于实际的List.然后，这里的flatten似乎没有做任何相关的事情.您不能只按以下方式操作吗?

Why are you calling toList and then flatten? If I'm reading the docs correctly for BSONDocument, the getAs method will return an Option for type T where in your case, you are specifying T as List[BSONObjectID]. When you call toList, you are basically unpacking the Option to either an empty List (in the case of a None) or into the actual List in the case of a Some. And then the flatten here doesn't seem to be doing anything relevant. Can't you just do it as:

doc.getAs[List[BSONObjectID]]("addresses")