如何对MongoDB集合中字段的不同值求和(使用猫鼬) [英] How to sum distinct values of a field in a MongoDB collection (utilizing mongoose)

问题描述

想象一下，我有一个名为 journals 的集合，其中包含如下文件:

Imagine I had a collection called journals containing documents like the following:

{
  "article": "id1",
  "d": 2
},
{
  "article": "id1",
  "d": 2
},
{
  "article": "id1",
  "d": 3
},
{
  "article": "id2",
  "d": 2
},
...

其中 d 是一种开关，而 article 是参考.现在我想得到如下结果:

Where d is kind of a switch and article is a reference. Now I want to have a result like the following:

[
  {
    "_id": "id1",
    "total": 3,
    "d2": 2,
    "d3": 1
  },
  {
    "_id": "id2",
    "total": 1,
    "d2": 1,
    "d3": 0
  }
]

我正在使用猫鼬，并且有一个名为 Journal 的模型.到目前为止，我得到的是……

I'm using mongoose and have a model called Journal. What I've got so far is…

Journal.aggregate(
  { $project: {
    articleId: 1,
    depth2 : { $gte:['$d', 2] },
    depth3 : { $eq:['$d', 3] }
  }},
  { $group: {
      _id: '$article',
      total: { $sum: 1 },
      d2: { $sum: '$depth2'},
      d3: { $sum: '$depth3'}
    }
  },
  function (err, journal) {
    console.log(journal);
  }
);

其结果是:

[
  {
    "_id": "id1",
    "total": 3,
    "d2": 0,
    "d3": 0
  },
  {
    "_id": "id2",
    "total": 1,
    "d2": 0,
    "d3": 0
  }
]

很显然，这里的错误是$eq:['$d', 3]不被求和，因为这会导致布尔值.

Obviously the error here is that $eq:['$d', 3] is not summed up because that results in a boolean.

那么，有没有更好的表达式可以将新的depth2和depth3字段投影到1或0而不是true或false?
还是有一个完整且更好的方法? :)

So is there a better expression that projects the new depth2 and depth3 fields to 1or 0 instead of true or false?
Or is there a complete and better approach? :)

我想避免进行3个查询并在匹配阶段之前添加{ $match: { d: 2 } }.

I'd like to avoid making 3 queries and prepending a matching phase like { $match: { d: 2 } }.

推荐答案

您可以使用 $cond 将布尔值转换为数值，但是您还得到了$gte，其中似乎是$eq应该是，而您的文档使用article代码使用articleId:

You can use $cond to convert a boolean to a numerical value, but you've also got a $gte where it seems an $eq should be and your docs use article while your code uses articleId:

Journal.aggregate([
  { $project: {
    article: 1,
    depth2 : { $cond: [{$eq: ['$d', 2]}, 1, 0] },
    depth3 : { $cond: [{$eq: ['$d', 3]}, 1, 0] }
  }},
  { $group: {
      _id: '$article',
      total: { $sum: 1 },
      d2: { $sum: '$depth2'},
      d3: { $sum: '$depth3'}
    }
  }
]);

输出:

{
    "result" : [ 
        {
            "_id" : "id2",
            "total" : 1,
            "d2" : 1,
            "d3" : 0
        }, 
        {
            "_id" : "id1",
            "total" : 3,
            "d2" : 2,
            "d3" : 1
        }
    ],
    "ok" : 1
}

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