Mongoose Aggregation to array of arrays

Question

I have a document structure like this:

{
  readings: [
    { t: 'temperature', r: 130 },
    { t: 'humidity', r: 100 }
  ],
  created_at: '2021-01-05T10:28:49.070Z'
},
{
  readings: [
    { t: 'temperature', r: 123 },
    { t: 'humidity', r: 456 }
  ],
  created_at: '2021-01-05T10:32:50.727Z'
}

I need to aggregate the documents for a particular reading type. For example, the result of aggregation for reading type with temperature should be like following:

[
  [130, '2021-01-05T10:28:49.070Z'],
  [123, '2021-01-05T10:32:50.727Z']
]

How can I do this with an aggregation pipeline or something?

Answer 1

You can't output an array of arrays from an aggregate pipe. But you can output an object with a single array field. Ie:

[
  {data: [130, '2021-01-05T10:28:49.070Z']},
  {data: [123, '2021-01-05T10:32:50.727Z']}
]

Here's a pipe that does just that https://mongoplayground.net/p/7UPyUICOncq

let result = await collection.aggregate([
    {$unwind: '$readings'},
    {$match: {
      'readings.t': 'temperature'
    }},
    {"$project": {
        _id: 0,
        data: ['$readings.r', '$created_at']
    }}
])

Then to reach your desired format just map the output

let formattedResult = result.map(({data}) => data)