Clojure - Monger操作符-缺少多字段连接和日期转换查询支持

Question

我有一个关于mongo的数据集，如:

{"month": 9, "year": 2015, "name": "Mr A"}
{"month": 9, "year": 2015, "name": "Mr B"}
{"month": 10, "year": 2015, "name": "Mr B"}
{"month": 11, "year": 2016, "name": "Mr B"}

我试图从这个using monger中获得最小日期，但是没有任何运气。

我能做的最好的就是用:

来区分月份和年份。

(mc/aggregate mongo-connection collection-name [{$group { :_id { :month "$month", :year "$year" } } }]))

给出如下结果:

[{"_id":{"year":2016,"month":11}},
 {"_id":{"year":2016,"month":10}},
 {"_id":{"year":2016,"month":9}}]

然后我使用clojure库来获得最小日期。有什么直接的方法吗?

Answer 1

在monger中，您可以先按年升序排序，然后按月升序排序，然后选择第一个结果

下面是文档中修改后的示例:

(ns my.service.server
  (:refer-clojure :exclude [sort find])
  (:require [monger.core :as mg]
            [monger.query :refer :all]))
(let [conn (mg/connect)
      db   (mg/get-db "your-db")
      coll "your-coll"]
  (with-collection db coll
    (find {})
    (fields [:year :month])
    ;; it is VERY IMPORTANT to use array maps with sort
    (sort (array-map :year 1 :month 1))
    (limit 1))

如果您经常这样做，请考虑在集合中添加索引以加快查询速度:

(ns my.app
  (:require [monger.core :as mg]
            [monger.collection :as mc]))
(let [conn (mg/connect)
      db   (mg/get-db "your-db")
      coll "your-collection"]
  ;; create an index on multiple fields (will be automatically named year_1_month_1 by convention)
  (mc/ensure-index db coll (array-map :year 1 :month 1)))