Question

鉴于以下文件......

{ "_id": ObjectId("56116d8e4a0000c9006b57ac"), "name": "Stock 1", "items" [
    { "price": 1.50, "desc": "Item 1" }
    { "price": 1.70, "desc": "Item 2" }
    { "price": 1.10, "desc": "Item 3" }
  ]
}

我希望得到价格最低的商品。以下是我尝试将价格从最低到最高排序：

db.stock.wip.aggregate([
  {$unwind: "$items"}, 
  {$sort: {"items.price":1}}, 
  {$group: {_id:"$_id", items: {$push:"$items"}}}
]);

......结果如下：

"result" : [
  {
    "_id" : ObjectId("56116d8e4a0000c9006b57ac"),
    "items" : [
      {
        "price" : 1.10,
        "desc" : "Item 3"
      },
      {
        "price" : 1.50,
        "desc" : "Item 1"
      },
      {
        "price" : 1.70,
        "desc" : "Item 2"
      }
    ]
  }
],
"ok" : 1

我如何获得第3项（最便宜的）？这是我正在寻找的结果：

{
  "price" : 1.10,
  "desc" : "Item 3"
}

Answer 1

使用$ first运算符而不是$ push，然后执行$ project：

db.stock.wip.aggregate([
  {$unwind: "$items"}, 
  {$sort: {"items.price":1}},
  {$group: {_id:"$_id", items: {$first:"$items"}}},
  { $project: {
    _id: 0,
    price: "$items.price",
    desc: "$items.desc",
  }}
]);

如果您想仅将结果限制在集合的第一项，请在$ sort后添加{$ limit：1}运算符：

db.stock.wip.aggregate([
  {$unwind: "$items"}, 
  {$sort: {"items.price":1}},
  {$limit:1},
  {$group: {_id:"$_id", items: {$first:"$items"}}},
  { $project: {
    _id: 0,
    price: "$items.price",
    desc: "$items.desc",
  }}
]);

Answer 2

我设法从您自己开始编写聚合查询，稍微更改了$group阶段并添加了额外的$project步骤：

db.stock.wip.aggregate([
  {$unwind: "$items"}, 
  {$sort: {"items.price":1}}, 
  {$group: {
    _id:null, 
    lowestItem: {$first:"$items"}}},
  {$project: {_id: 0, price: "$lowestItem.price", desc: "$lowestItem.desc"}}
]);

MongoDB：如何获取包含最低值的子文档

2 个答案: