将我的数据转换为棘手的格式时遇到了一些麻烦。
文档具有以下字段： subject, importance [high,medium,low]
样本文件：
{_id: "", subject: "red", importance: "high"}
我喜欢返回看起来像这样的数据：
[{_id: subject, high: 5, medium: 6, low: 3}]
数字与每个重要性级别对应的文档数相对应
这是我到目前为止所拥有的：
{
$group: {
_id: {subject: "$subject", importance: "$importance"},
count: {$sum: 1},
}
},
{
$group: {
_id: "$_id.subject",
data: {
$push: {
importance: "$_id.importance", count: "$count"
}
}
}
}
有没有一种方法可以使用的值$_id.importance并将其作为键并具有$count作为值？
您可以汇总如下：
$group通过subject和importance，获得各自的计数。$project它将相对于importance字段可以容纳的选项数量线性增长。目前是三- high，low和medium。$group再次返回结果，subject并使用$sum运算符对accumulate重要性字段的不同值进行计数。样例代码：
db.t.aggregate([
{$group:{"_id":{"subject":"$subject",
"importance":"$importance"},
"count":{$sum:1}}},
{$project:{"_id":0,
"subject":"$_id.subject",
"result":{$cond:[
{$eq:["$_id.importance","high"]},
{"high":"$count"},
{$cond:[{$eq:["$_id.importance","low"]},
{"low":"$count"},
{"medium":"$count"}]}]}}},
{$group:{"_id":"$subject",
"low":{$sum:"$result.low"},
"medium":{$sum:"$result.medium"},
"high":{$sum:"$result.high"}}},
])
测试数据：
db.t.insert([
{"subject":"history","importance":"high"},
{"subject":"geography","importance":"low"},
{"subject":"history","importance":"low"},
{"subject":"history","importance":"medium"},
{"subject":"geography","importance":"low"},
{"subject":"history","importance":"low"}
])
结果：
{ "_id" : "geography", "low" : 2, "medium" : 0, "high" : 0 }
{ "_id" : "history", "low" : 2, "medium" : 1, "high" : 1 }
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