当在mongodb中聚合时,$对字段做什么

Question

db.cata.aggregate([{"$unwind":"$review"},{$group:{_id:"review",cnt:{$sum:1}}}]).pretty()

db.cata.aggregate([{"$unwind":"$review"},{$group:{_id:"$review",cnt:{$sum:1}}}]).pretty()

这两个查询在document..how $ review和review do中的作用... $如何使用字段...

我的文件是

{
"_id" : ObjectId("56dd01bdf3b660327b932da1"),
"product" : "super",
"price" : 10,
"review" : [
    {
        "user" : "fred",
        "comment" : "great",
        "rating" : 10
    },
    {
        "user" : "tom",
        "comment" : "i agree",
        "rating" : 3
    },
    {
        "user" : "vin",
        "comment" : "good",
        "rating" : 9
    },
    {
        "user" : "anubhav",
        "comment" : "too good",
        "rating" : 8
    }]
}

Answer 1

根据Mongodb 文档所说

操作数是字段路径：

{ $unwind: <field path> }

要指定字段路径，请在字段名称前添加美元符号$并用引号引起来。