flatten nested array in mongodb

//here is document with NESTED Array
{
    "_id": "xyz-800",
    "site": "xyz",
    "user": 800,
    "timepoints": [
        {"timepoint": 0, "a": 1500, "b": 700},
        {"timepoint": 2, "a": 1000, "b": 200},
        {"timepoint": 4, "a": 3500, "b": 1500}
    ]
}

/**
  Starting MongoDb 3.4, we can use $addFields to add the top level fields 
  to the embedded document and use $replaceRoot to promote 
  embedded document to top level.
*/

db.records.aggregate({
    $unwind: "$timepoints"
}, {
    $addFields: {
        "timepoints._id": "$_id",
        "timepoints.site": "$site",
        "timepoints.user": "$user"
    }
}, {
    $replaceRoot: {
        newRoot: "$timepoints"
    }
})

// Sample Output

{ "timepoint" : 0, "a" : 1500, "b" : 700, "_id" : "xyz-800", "site" : "xyz", "user" : 800 }
{ "timepoint" : 2, "a" : 1000, "b" : 200, "_id" : "xyz-800", "site" : "xyz", "user" : 800 }
{ "timepoint" : 4, "a" : 3500, "b" : 1500, "_id" : "xyz-800", "site" : "xyz", "user" : 800 }

You also can use $project in aggregation framework.

In this case it would be:

"$project": {
          "_id": 1,
          "site": 1,
          "user": 1,
          "timepoint": "$timepoints.timepoint",
          "a": "$timepoints.a",
          "b": "$timepoints.b",
    }