Mongo聚合笔记

mongo很难用, 主要是因为不习惯写很多字

文档写得很不错.

https://docs.mongodb.com/manual/reference/aggregation-variables/#agg-system-variables

https://docs.mongodb.com/manual/reference/operator/aggregation/sort/index.html

https://docs.mongodb.com/manual/reference/operator/aggregation/group/index.html

这篇写得也还可以.

https://www.jianshu.com/p/206f036bdfc5

对应springboot 中mongtemplate, 就是这样写

这个写得很不错

https://kb.objectrocket.com/mongo-db/mongodb-group-by-multiple-fields-using-aggregation-function-464

db.sales.aggregate(
   [
     { $sort: { item: -1, date: -1 } },
     {
       $group:
         {
           _id: "$item",
           firstSalesDate: { $first: "$date" },
           firstId: { $first: "$_id" },
           firstRoot: { $first: "$$ROOT" },
         }
     }
   ]
)

　　

Criteria criteria = Criteria.where("xxx").is("xxxx").and("xxx").is(xxx);        
Aggregation aggregation = Aggregation.newAggregation(
                Aggregation.match(criteria),
                Aggregation.sort(Sort.Direction.ASC, "createTime"),
                Aggregation.group("xx", "xx")
                        .first("$id").as("xx")
                        .first("$xxx").as("xx")
                        .first("$xxx").as("xx")
                        .first("$xxxx").as("xx")
                        .first("$xxxx").as("xx")
                        .first("$xxxx").as("xx")
        );

stackflow上的一段
--------------------------------------------------------------------------

Aggregation agg = newAggregation( //
match(where("hotelCode").is("0360")), //
sort(Direction.DESC, "confirmationNumber", "timestamp"), //
group("confirmationNumber") //
.first("timestamp").as("timestamp") //
.first(Aggregation.ROOT).as("reservationImage") //
);

--------------------------------------------------------------------------

比较坑的是, $_id在 mongotemplate里叫做$id, 少了_, 还有 Aggregation.ROOT, Aggregation.CURRENT