MongoDB：如果存在，则替换整个阵列；如果不存在，则创建新阵列'；不存在

如果我的集合中充满了如下所示的数组

{
        "_id" : ObjectId("51f21c47564f4a9f411baf8d"),
        "Gender" : [
                {
                        "key" : "m",
                        "desc" : "male"
                }
        ]
}
{
        "_id" : ObjectId("51f21c9a564f4a9f411baf8e"),
        "marital status" : [
                {
                        "key" : "m",
                        "desc" : "married"
                },
                {
                        "key" : "s",
                        "desc" : "single"
                }
        ]
}

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如果整个数组存在，则用什么查询替换它？如果不存在，则用什么查询创建新数组？

语法：

db.collection.update({<key>: {'$exists': true}}, {<key>: <value>}, 
{upsert: true});

db.collection.update({"Gender": {'$exists': true}}, {"Gender": 
[{"key": "m", "desc": "male"}, {"key": "f", "desc": "female"}]}, true);