mongodb - 如何在 MongoDB 中对数组类型的字段进行排序

Question

我正在尝试编写一个查询，该查询将以升序对 special_dates 数组进行排序，然后根据 MongoDB 中数组中的第一个日期进行排序。

我有以下数据:

/* 1 */
{
    "_id" : ObjectId("5f9c0dfaf88d0a318ad826b1"),
    "holiday_for" : "range",
    "specific_dates" : [ 
        "2020-10-30", 
        "2020-10-31", 
        "2020-11-01", 
        "2020-11-02", 
        "2020-11-03", 
        "2020-11-04", 
        "2020-11-05", 
        "2020-11-06", 
        "2020-11-07", 
        "2020-11-08", 
        "2020-11-09", 
        "2020-11-10", 
        "2020-11-11", 
        "2020-11-12", 
        "2020-11-13", 
        "2020-11-14", 
        "2020-11-15", 
        "2020-11-16", 
        "2020-11-17", 
        "2020-11-18", 
        "2020-11-19", 
        "2020-11-20", 
        "2020-11-21", 
        "2020-11-22", 
        "2020-11-23", 
        "2020-11-24", 
        "2020-11-25", 
        "2020-11-26", 
        "2020-11-27", 
        "2020-11-28", 
        "2020-11-29", 
        "2020-11-30", 
        "2020-12-01", 
        "2020-12-02", 
        "2020-12-03", 
        "2020-12-04", 
        "2020-12-05", 
        "2020-12-06", 
        "2020-12-07", 
        "2020-12-08", 
        "2020-12-09", 
        "2020-12-10", 
        "2020-12-11", 
        "2020-12-12", 
        "2020-12-13", 
        "2020-12-14", 
        "2020-12-15", 
        "2020-12-16", 
        "2020-12-17", 
        "2020-12-18", 
        "2020-12-19", 
        "2020-12-20", 
        "2020-12-21", 
        "2020-12-22", 
        "2020-12-23", 
        "2020-12-24", 
        "2020-12-25"
    ],
    "description" : "Add holiday",
    "location_id" : 0
}

/* 2 */
{
    "_id" : ObjectId("5fa2220abb17c6473c2f4368"),
    "holiday_for" : "specific",
    "specific_dates" : [ 
        "2020-10-31", 
        "2020-11-02", 
        "2020-10-30"
    ],
    "description" : "Add holiday",
    "location_id" : 0
}

/* 3 */
{
    "_id" : ObjectId("5fc7591a3b1b0175500d07ea"),
    "holiday_for" : "specific",
    "specific_dates" : [ 
        "2020-12-05", 
        "2020-12-02", 
        "2020-12-01"
    ],
    "location_id" : 0
}

我需要根据 special_dates 数组中的第一个日期对这些文档进行排序:

db.getCollection('holidays').aggregate([
{$unwind: "$specific_dates"},
{$sort: {"specific_dates": -1}},
{$group: {
    "_id": "$_id", 
    "holiday_for": {$first: "$holiday_for"}, 
    "description": {$first: "$description"},
    "location_id": {$first: "$location_id"},
    "specific_dates": {$push: "$specific_dates"}}},
])

但这给出了以下意想不到的结果。有人可以指导我如何执行此操作吗？

Answer 1

无需$unwind，您可以使用$first从数组中获取第一个元素，

db.collection.aggregate([
  { $addFields: { first_date: { $first: "$specific_dates" } } },
  { $sort: { first_date: -1 } }
])

Playground

---

第二个选项您可以使用$arrayElemAt，

db.collection.aggregate([
  { $addFields: { first_date: { $arrayElemAt: ["$specific_dates", 0] } } },
  { $sort: { first_date: -1 } }
])

Playground

---

根据您的评论的第三个选项，首先对该数组进行排序，

• $reduce 迭代使用 $setUnion、$concatArrays 按升序排序的 special_dates 数组的循环> 第一个当前对象和第二个初始值，它将改变数组的顺序为降序
• 接下来的两个阶段保持不变

db.collection.aggregate([
  {
    $addFields: {
      specific_dates: {
        $reduce: {
          input: { $setUnion: "$specific_dates" },
          initialValue: [],
          in: { $concatArrays: [["$$this"], "$$value"] }
        }
      }
    }
  },
  { $addFields: { first_date: { $arrayElemAt: ["$specific_dates", 0] } } },
  { $sort: { first_date: -1 } }
])

Playground