$unwind double nested array for aggregation

Question

I'm trying to set up a time series in mongodb (using this as a guide). Let's say that I have a collection where each document looks like this:

{
  _id: '...',
  timestamp_minute: '2018-01-01T12:30:00.000Z',
  numbers: [[1, 2, ..., 10], ..., [1, 2, ..., 10]],
  letters: [[a, b, ..., j], ..., [a, b, ..., j]]
}

How can I unwind it so it looks like this:

[
  {_id: '...', timestamp_minute: '2018-01-01T12:30:00.000Z', values: 1, letters: a},
  {_id: '...', timestamp_minute: '2018-01-01T12:30:00.000Z', values: 2, letters: b},
  {_id: '...', timestamp_minute: '2018-01-01T12:30:00.000Z', values: 3, letters: c},
  ...
]

The data is stored on a document-per-minute basis with tenth-of-a-second precision. That means that the first level of values is usually length 60 (the seconds in a minute), and each nested one is length 10 (the tenth-of-a-second in a second).

I want to be able to unwind it in order to generate aggregated data.

s7vr · Accepted Answer · 2018-07-16 02:51:23Z

2

You can try below aggregation in 3.4 version and more.

db.colname.aggregate([
  {"$project":{
    "timestamp_minute":1,
    "numbersandletters":{
      "$map":{
        "input":{"$range":[0,{"$size":"$numbers"}]},
        "as":"ix",
        "in":{
          "$zip":{
            "inputs":[
              {"$arrayElemAt":["$numbers","$$ix"]},
              {"$arrayElemAt":["$letters","$$ix"]}
            ]
          }
        }
      }
    }
  }},
  {"$unwind":"$numbersandletters"},
  {"$unwind":"$numbersandletters"},
  {"$project":{
    "timestamp_minute":1,
    "values":{"$arrayElemAt":["$numbersandletters",0]},
    "letters":{"$arrayElemAt":["$numbersandletters",1]}
  }}
])

answered Jul 16, 2018 at 2:51

s7vr

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1 Comment

danielrs Over a year ago

Really smart way of solving the issue. Learned a lot just by reading this. Thank you!

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