MongoEngine中如何实现嵌套查询?

{
    "_id" : ObjectId("56d56366806ea50fbcdf4f7e"),
    "mac" : [ 
        "cc1be0701fa5"
    ],
    "shop_id" : 3307,
    "reported_info" : [ 
        {
            "reported_time" : ISODate("2016-03-01T17:39:50.844Z"),
            "online_mac" : [ 
                "mac1", 
                "mac2", 
                "mac3", 
                "mac4", 
                "mac5"
            ]
         }
}

对于这样的数据库结构,如果要查询在shop_id=3307的文档下,在reported_info里有没有online_mac为mac1的记录,该如何查询?