How can I retrieve specific element from array using aggregation in Mongoose NodeJS

Question

So I have a code for the aggregation

const documents = await deviceCollection
  .aggregate([
    {
      $match: {
        'readings.t': sensorType,
      },
    },
    { $unwind: '$readings' },
    {
      $project: {
        _id: 0,
        data: ['$readings.r', '$created_at'],
      },
    },
  ])
  .toArray();

return documents.map(({ data }) => data);

and I have a document structure like this one

{
    "readings" : [ 
        {
            "t" : "temperature",
            "r" : 6
        }, 
        {
            "t" : "humidity",
            "r" : 66
        }
    ],
    "created_at" : ISODate("2021-02-24T09:45:09.858Z"),
    "updated_at" : ISODate("2021-02-24T09:45:09.858Z")
}

I need to aggregate r value and created_at as UTC number for a particular reading type in a date range. For example, the expected output for temperature reading is:

[
 [6, 1616061903204],
 [5.6, 1616061903204]
]

But the code returns this

[
    [
        6,
        "2021-02-24T09:45:09.858Z"
    ],
    [
        66,
        "2021-02-24T09:45:09.858Z"
    ],
    [
        5.6,
        "2021-02-24T09:50:09.820Z"
    ],
    [
        68,
        "2021-02-24T09:50:09.820Z"
    ],
]

And it means that I get the humidity type value as well.

Answer 1

• $match your condition
• $unwind deconstruct readings array
• $match again to filter readings object
• $toLong to convert ISO date format to timestamp
• $group by null and construct readings array in a single array

const documents = await deviceCollection.aggregate([
  { $match: { "readings.t": sensorType } },
  { $unwind: "$readings" },
  { $match: { "readings.t": sensorType } },
  {
    $project: {
      readings: [
        "$readings.r",
        { $toLong: "$created_at" }
      ]
    }
  },
  {
    $group: {
      _id: null,
      readings: { $push: "$readings" }
    }
  }
]).toArray();

return documents.length ? documents[0].readings : [];

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