Project elements from Array of Arrays in MongoDB

Question

I am currently struggling with a search through an array of arrays within a JSON file that is imported into MongoDB compass. At the code below you can see the array "track" contains multiple other arrays. Now I want to get the 4th value of each of this arrays, for example, I want to get "load" from the first, "mouseover" from the second and so on...

{
"name":"nutzer1_50ms",
"ip":"::1",
"date":"Sat, 16 Dec 2017 21:53:19 +0100",
"screen":"1920x1080",
"window":"1920x966",
"document":"1024x786",
"track":[
[1513457569930,0,0,"load",""],
[1513457569953,79,229,"mouseover","p3"],
[1513457570274,79,228,"mousemove","p3"],
[1513457570280,79,226,"mousemove","p3"],
["end","end","end","end"]]
}

I would be very glad to get some help on this :)

s7vr · Accepted Answer · 2017-12-17 00:38:25Z

1

You can try below aggregation query.

$map iterating over track values & $arrayElemAt to project the 4th element from array.

db.collection_name.aggregate([
  {
    "$project": {
      "4th": {
        "$map": {
          "input": "$track",
          "as": "track",
          "in": {
            "$arrayElemAt": [
              "$$track",
              3
            ]
          }
        }
      }
    }
  }
])

answered Dec 17, 2017 at 0:38

s7vr

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7 Comments

Andrew Bishop Over a year ago

thx first of all, I'm trying to use this code but it won´t work :(

s7vr Over a year ago

np. Can I see your query ? Which part is not working ?

Andrew Bishop Over a year ago

I am taking this straight into MongDB Compass Filter: {"$project": {"4th": {"$map": {"input": "$track","as": "track","in": {"$arrayElemAt": ["$$track",3]}}}}}

s7vr Over a year ago

Query Filter seems like for a regular query. This is a aggregate query. Not sure where/how run aggregate query in compass.

Andrew Bishop Over a year ago

Ok thanks anyway, I need to get the data visualized there :)

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