MongoDB - 从avg聚合的结果中获取最大值

Question

我收集了产品,这些产品都有评估.我需要选择具有最高评估平均值的产品.问题是我可以按平均分组产品但我无法按平均分组并选择平均值最高的产品.

要重现我的问题,请按照下列步骤操作:

插入产品:

db.products.insert([
{
    name: "Product1",
    price: 1000,
    features: {
        feature1: 0.8,
        feature2: 23
    },
    tags: ["tag1", "tag2", "tag3", "tag4"],
    assessments: [
        {name: "John", assessment: 3},
        {name: "Anna", assessment: 4},
        {name: "Kyle", assessment: 3.6}
    ] 
},
{
    name: "Product2",
    price: 1200,
    features: {
        feature1: 4,
        feature2: 4000,
        feature3: "SDS"
    },
    tags: ["tag1"],
    assessments: [ 
        {name: "John", assessment: 5},
        {name: "Richard", assessment: 4.8}
    ]
},
{
    name: "Product3",
    price: 450,
    features: {
        feature1: 1.3,
        feature2: 60
    },
    tags: ["tag1", "tag2"],
    assessments: [ 
        {name: "Anna", assessment: 5},
        {name: "Robert", assessment: 4},
        {name: "John", assessment: 4},
        {name: "Julia", assessment: 3}
    ]
},
{
    name: "Product4",
    price: 900,
    features: {
        feature1: 1700,
        feature2: 17
    },
    tags: ["tag1", "tag2", "tag3"],
    assessments: [ 
        {name: "Monica", assessment: 3},
        {name: "Carl", assessment: 4}
    ]
}
])

我希望通过平均评估进行分组,并选择最高平均值的产品.

我这样做:

db.products.aggregate([
    { $unwind : "$assessments" },
    { $group: 
        {
            _id: "$name",
            avg_assessment: {$avg: "$assessments.assessment"}
        }
    },
    { $project:
        {
            _id: 0,
            product: "$_id",
            avg_assessment: 1
        }
    }
]) 

此查询的结果是:

{ "avg_assessment" : 3.5, "product" : "Product4" }
{ "avg_assessment" : 4, "product" : "Product3" }
{ "avg_assessment" : 4.9, "product" : "Product2" }
{ "avg_assessment" : 3.533333333333333, "product" : "Product1" }

尼斯.然后我尝试使用以下查询选择具有最高平均值的产品:

db.products.aggregate([
        { $unwind : "$assessments" },
        { $group: 
            {
                _id: "$name",
                avg_assessment: { $max: {$avg: "$assessments.assessment"}}
            }
        },
        { $project:
            {
                _id: 0,
                product: "$_id",
                avg_assessment: 1
            }
        }
    ]) 

但结果是相同的,但有四舍五入的值:

{ "avg_assessment" : 4, "product" : "Product4" }
{ "avg_assessment" : 5, "product" : "Product3" }
{ "avg_assessment" : 5, "product" : "Product2" }
{ "avg_assessment" : 4, "product" : "Product1" }

这是怎么回事？哪里有问题？

Answer 1

您可以尝试以下聚合.没有$unwind此需要.

计算$avg每个评估,然后进行排序.

$group与$first挑选具有最高平均价值的评估.

添加$project阶段以限制字段.

db.products.aggregate([
  { "$addFields" : {"avg_assessment":{"$avg":"$assessments.assessment" }}},
  { "$sort":{"avg_assessment":-1}},
  { "$group": 
    {
       "_id": null,
       "highest_avg_assessment": { $first:"$$ROOT"}
     }
   }
])