Mongodb 聚合Aggregation中$sum:1 后面的1是什么意思

源数据:

{
    "_id" : "GuqXmAkkARqhBDqhy",
    "beatmapset_id" : "342537",
    "version" : "MX",
    "diff_approach" : "5",
    "artist" : "Yousei Teikoku",
    "title" : "Kokou no Sousei",
    "difficultyrating" : "3.5552737712860107"
}
{
    "_id" : "oHLT7KqsB7bztBGvu",
    "beatmapset_id" : "342537",
    "version" : "HD",
    "diff_approach" : "5",
    "artist" : "Yousei Teikoku",
    "title" : "Kokou no Sousei",
    "difficultyrating" : "2.7515676021575928"
}
{
    "_id" : "GbotZfrPEwW69FkGD",
    "beatmapset_id" : "342537",
    "version" : "NM",
    "diff_approach" : "5",
    "artist" : "Yousei Teikoku",
    "title" : "Kokou no Sousei",
    "difficultyrating" : "0"
}

 然后运行以下的命令:
 

db.getCollection('dup_case').aggregate(
[
{$group:{
    _id:{diff_approach:'$diff_approach'},
    count:{$sum:2}
    }},
{$match:{count:{$gt:1}}}
]
)

 
 
返回的count是6
 
所以
$sum:1 的含义:
如果前面的情况出现一次,就加1, 如果后面$sum:2 那么每次前面条件满足一次就加2