如何计算所有选定数据集的平均时间值？

Question

There are multiple documents in my db with this structure:

{
    "_id" : ObjectId("5d7f6a937563a63c1d8b4639"),
    "admission" : ISODate("2019-09-16T10:27:20.197Z"),
    "target" : [
        {
            "score" : 3
        },
    {
            "score" : 2
        }
    ],
    "contact" : {
        "user" : "user",
        "time" : ISODate("2019-09-16T10:47:20.197Z")
    }
}

I need to select all datasets with the score of 3 as a first element of target and return the average time difference between admission and contact.time .

So the result should be something like: 342 datasets with score of 3 have an average time difference of 25.34 minutes

So the first part should be easy by doing

db.data.find({ 'target.0': 2 })

But how do I get the avarage time difference from all these selected datasets?

Answer 1

You can use Aggregation Pipeline to do this. Based on your data format, you need target.0.score instead of target.0

db.data.aggregate([
{
    $match : {"target.0.score": 3} // Filter All targets
},
{
    $group : {
        "_id" : null,
        "Datasets" : {$sum : 1},
        "Average" : {"$avg" : {$divide: [{$subtract: ["$contact.time", "$admission"]}, 60000]}} // Calculate Average from distance, also, Divide by 60000 to convert from milliseconds to seconds. 
    }
}
])

The result document will be as follows:

{
    "_id" : null,
    "Datasets" : 3.0,
    "Average" : 26.6666666666667
}