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请问 .aggregate操作中的 project，可以产生新字段，将其他两个字段的值拼接在一起吗？ 该怎么写？
@sevengong
使用 $concat
$concat:["$lastname",",","$firstname"]
与 萌阔论坛 的连接断开，我们正在尝试重连，请耐心等待
