mongodb查询聚合组中的子集合计数

我有一个mongo新手问题

我有一个汽车系列，它有一个功能阵列 我正试图按品牌对汽车进行分组，并总结该品牌的所有功能

这是一个类比，因为我正在研究的是一个具有类似问题的金融应用程序，所以

收藏下列文件：

/* 1 */
{
    "_id" : ObjectId("5ad870ed22b6ac63f3b66359"),
    "make" : "toyota",
    "model" : "corolla",
    "year" : 1992,
    "type" : "sedan",
    "features" : []
}

/* 2 */
{
    "_id" : ObjectId("5ad8712222b6ac63f3b66367"),
    "make" : "toyota",
    "model" : "camry",
    "year" : 2014,
    "type" : "sedan",
    "features" : [ 
        "cruise control", 
        "air conditioning", 
        "auto headlights"
    ]
}

/* 3 */
{
    "_id" : ObjectId("5ad8714122b6ac63f3b6636c"),
    "make" : "toyota",
    "model" : "celica",
    "year" : 2003,
    "type" : "sports hatch",
    "features" : [ 
        "cruise control", 
        "air conditioning", 
        "turbo"
    ]
}

/* 4 */
{
    "_id" : ObjectId("5ad8733722b6ac63f3b663a9"),
    "make" : "mazda",
    "model" : "323",
    "year" : 1998,
    "type" : "sports hatch",
    "features" : [ 
        "powered windows", 
        "air conditioning"
    ]
}

/* 5 */
{
    "_id" : ObjectId("5ad8738022b6ac63f3b663af"),
    "make" : "mazda",
    "model" : "3",
    "year" : 2014,
    "type" : "sports hatch",
    "features" : [ 
        "powered windows", 
        "air conditioning", 
        "cruise control", 
        "navigation"
    ]
}

/* 6 */
{
    "_id" : ObjectId("5ad873b322b6ac63f3b663b6"),
    "make" : "mazda",
    "model" : "cx9",
    "year" : 2012,
    "type" : "sports utility vehicle",
    "features" : [ 
        "powered windows", 
        "air conditioning", 
        "cruise control", 
        "navigation", 
        "4 wheel drive", 
        "traction control"
    ]
}

我想把汽车按品牌分类，并清点所有的零件

db.getCollection('cars').aggregate([
{
    $match : 
    { 
        $or : [{ make : "toyota"}, { make : "mazda"}]
    }       
},

{
  $group: { _id: '$make', count: { $sum: { $count : "$features" } } },
}

])

我不能让$count以这种方式工作，只计算每个分组项目的功能

---

建议？

我想你可以先按

make

，

sum

对

功能的长度进行分组，如下所示：

db.getCollection('myCollection').aggregate([

   { "$group": { "_id": "$make", "count": { "$sum": { "$size": "$features" } } } }

])

为了更好地理解，请发布数据集。