unicloud子集合去重统计

unicloud的子集合去重统计到底要怎么整呢，太困惑的，请懂的大侠指教一下，如下面集合，统计每个分组userid的实有个数，mongodb实在没怎么学，见笑了

{
"requestId": "1796c10f5b6_1",
"data": [{
"_id": "28ee4e3e6096568b16e8dd363856cedd",
"title": "测试问题一大堆",
"status": 1,
"djsl": 259,
"userid": ["o9YyG4lHDMbXn4RhgBLrj7ZQzEnY", "o9YyG4lHDMbXn4RhgBLrj7ZQzEnY",
"o9YyG4lHDMbXn4RhgBLrj7ZQzEnY", "o9YyG4lHDMbXn4RhgBLrj7ZQzEnY", "o9YyG4lHDMbXn4RhgBLrj7ZQzEnY",
"o9YyG4lHDMbXn4RhgBLrj7ZQzEnY", "o9YyG4lHDMbXn4RhgBLrj7ZQzEnY", "o9YyG4lHDMbXn4RhgBLrj7ZQzEnY",
"o9YyG4lHDMbXn4RhgBLrj7ZQzEnY", "o9YyG4lHDMbXn4RhgBLrj7ZQzEnY", "o9YyG4lHDMbXn4RhgBLrj7ZQzEnY",
"o9YyG4lHDMbXn4RhgBLrj7ZQzEnY", "o9YyG4lHDMbXn4RhgBLrj7ZQzEnY", "o9YyG4lHDMbXn4RhgBLrj7ZQzEnY",
"o9YyG4lHDMbXn4RhgBLrj7ZQzEnY", "081AfE0w3AmplW2Agq0w3es9nq3AfE0F",
"081AfE0w3AmplW2Agq0w3es9nq3AfE0F", "001lFKFa1bN31B0HymJa1tjO5o3lFKFg",
"001lFKFa1bN31B0HymJa1tjO5o3lFKFg", "001lFKFa1bN31B0HymJa1tjO5o3lFKFg",
"041ZRZ0004pMFL1sX1200ZbtJh4ZRZ0a", "041ZRZ0004pMFL1sX1200ZbtJh4ZRZ0a",
"041ZRZ0004pMFL1sX1200ZbtJh4ZRZ0a", "041ZRZ0004pMFL1sX1200ZbtJh4ZRZ0a",
"041ZRZ0004pMFL1sX1200ZbtJh4ZRZ0a", "0519djGa1O5t0B0iYHFa1pX5XV09djGE",
"011sMa000xyBGL1gwn300MIoDs3sMa0D", "011sMa000xyBGL1gwn300MIoDs3sMa0D",
"011sMa000xyBGL1gwn300MIoDs3sMa0D", "02199LFa12111B0c0HJa1ZrMZN099LFF",
"02199LFa12111B0c0HJa1ZrMZN099LFF", "",
"", "031PZLFa1ff21B0nHMFa1pCNNe0PZLFN",
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]
}, {
"_id": "79550af2609ca8fe165ca0374a321283",
"title": "测试主题11111",
"status": 1,
"djsl": 5,
"userid": ["o9YyG4ufnusetxpmnS51WR9dRw28", "o9YyG4ufnusetxpmnS51WR9dRw28",
"o9YyG4ufnusetxpmnS51WR9dRw28",
"o9YyG4ufnusetxpmnS51WR9dRw28", "o9YyG4lHDMbXn4RhgBLrj7ZQzEnY"]
}, {
"_id": "b00064a7609ca90416f8c51811c6cf6c",
"title": "测试主题222",
"status": 1,
"djsl": 3,
"userid": ["o9YyG4ufnusetxpmnS51WR9dRw28", "o9YyG4ufnusetxpmnS51WR9dRw28",
"o9YyG4ufnusetxpmnS51WR9dRw28"]
}, {
"_id": "28ee4e3e609d1be11852251a58b49349",
"title": "测试发卷",
"djsl": 0,
"userid": []
}]
}