如何在MongoDB中对每个组内max对应的文档进行分组和选择？

这是我的 mongo 收藏“销售”：

{"title":"Foo", "hash": 17, "num_sold": 49, 
"place": "ABC"}

{"title":"Bar", "hash": 18, "num_sold": 55, 
"place": "CDF"}

{"title":"Baz", "hash": 17, "num_sold": 55,
"place": "JKN"}

{"title":"Spam", "hash": 17, "num_sold": 20,
"place": "ZSD"}

{"title":"Eggs", "hash": 18, "num_sold": 20, 
"place": "ZDF"}

我想按哈希分组并返回具有最大“num_sold”的文档。所以作为输出我想看到：

{"title":"Baz", "hash": 17, "num_sold": 55,
    "place": "JKN"}

 {"title":"Bar", "hash": 18, "num_sold": 55, 
    "place": "CDF"}

我知道聚合运算符的基础知识，这是我如何分组并获取 num_sold 最大值的方法，但我需要与最大值相对应的整个文档，而不仅仅是值。

db.getCollection('sales').aggregate([
{$group: {_id: "$hash", max_sold : {$max: '$value'}}}
])

在 SQL 中我会用 join 来完成，但在 mongo 中。我还读到，在 mongo 中，组和排序不能很好地协同工作。

您可以使用$redact阶段来实现这一点。它避免了使用$sort然后再次做一个$group or an $unwind.

• $group by _id并得到最大值max_num_sold对于每个组，使用以下方法累积该组中的所有文档$push操作员。
• $redact到每组的子文档中，只保留那些具有最大max_num_sold在他们的num_sold

示例代码：

db.getCollection('sales').aggregate([
{$group:{"_id":"$hash",
         "max_num_sold":{$max:"$num_sold"},
         "records":{$push:"$$ROOT"}}},
{$redact:{$cond:[{$eq:[{$ifNull:["$num_sold","$$ROOT.max_num_sold"]},
                       "$$ROOT.max_num_sold"]},
                "$$DESCEND","$$PRUNE"]}},
])

测试数据：

db.getCollection('sales').insert([
{"title":"Foo","hash":17,"num_sold":49,"place":"ABC"},
{"title":"Bar","hash":18,"num_sold":55,"place":"CDF"},
{"title":"Baz","hash":17,"num_sold":55,"place":"JKN"},
{"title":"Spam","hash":17,"num_sold":20,"place":"ZSD"},
{"title":"Eggs","hash":18,"num_sold":20,"place":"ZDF"}
])

测试结果：

{
        "_id" : 18,
        "max_num_sold" : 55,
        "records" : [
                {
                        "_id" : ObjectId("567874f2b506fc2193a22696"),
                        "title" : "Bar",
                        "hash" : 18,
                        "num_sold" : 55,
                        "place" : "CDF"
                }
        ]
}
{
        "_id" : 17,
        "max_num_sold" : 55,
        "records" : [
                {
                        "_id" : ObjectId("567874f2b506fc2193a22697"),
                        "title" : "Baz",
                        "hash" : 17,
                        "num_sold" : 55,
                        "place" : "JKN"
                }
        ]
}