router.get('/foodList', (req, res) => {
  Classes.aggregate([{
    $lookup: {
      from: "foods",
      localField: "_id",
      foreignField: "class_id",
      as: "foods"
    }
  }, {
    $sort: {
      date: 1
    }
  }, {
    $group: {
      _id: "$_id",
      className: {
        $first: '$className'
      },
      foods: {
        $first: '$foods'
      },
    }
  }, {
    $project: {
      'foods.class_id': 0,
    }
  }], ((err, doc) => {
    res.json({
      code: 200,
      message: "查询成功！",
      data: doc
    })
  }))
})

{
    "code": 200,
    "message": "查询成功！",
    "data": [
        {
            "_id": "5fc9c5a18fdde1189511ec43",
            "className": "热销",
            "foods": [
                {
                    "_id": "5fcb96af79144604657d049a",
                    "shop_id": "5fbd20455e2f15606b946828",
                    "foodName": "芝麻酱",
                    "foodImg": "http://erkong.ybc365.com/67315202012052214277383.png",
                    "foodPrice": "3",
                    "desc": "主要原料：金针菇",
                    "date": "2020-12-05T14:18:23.300Z",
                    "__v": 0
                }
            ]
        },
        {
            "_id": "5fcb99c4b1d98f04e55ea516",
            "className": "素菜",
            "foods": [
                {
                    "_id": "5fcb99e1b1d98f04e55ea517",
                    "shop_id": "5fbd20455e2f15606b946828",
                    "foodName": "平菇",
                    "foodImg": "http://eog.ybc365.co/6731502012052214277383.png",
                    "foodPrice": "3",
                    "desc": "主要原料：金针菇",
                    "date": "2020-12-05T14:32:01.574Z",
                    "__v": 0
                },
                {
                    "_id": "5fcb9a35ff27e804f49245b1",
                    "shop_id": "5fbd20455e2f15606b946828",
                    "foodName": "海鲜菇",
                    "foodImg": "http://eog.ybc365.cm/61315202012052214277383.png",
                    "foodPrice": "3",
                    "desc": "主要原料：海鲜菇",
                    "date": "2020-12-05T14:33:25.921Z",
                    "__v": 0
                }
            ]
        }
    ]
}