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mongo不通过forEach如何更新同文档字段?

假设MONGO文档中有xxoo(数组)和length(数组大小)两个字段。
不通过find.foreach(update)遍历的方式,如何更新length为xxoo数组的大小?
?示例文档:

{
    "_id": ObjectId("5f8554f49a8c7ff8d844555f"),
    "xxoo": ["x", "x", "o", "o"],
    "length": NumberInt("0")
    ……
}

?期望方法:
db.getCollection("xxx").update({'_id': '5f8554f49a8c7ff8d844555f'},{$set: {'length': {$size: $xxoo}})


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