MongoDB - Aggregate - Filter on the last element in an array field in collection

Question

I have a Devices collection in MongoDB with the following structure:

{
  "group": [
    "group1"
  ]
},
{
  "group": [
    "group1",
    "group2"
  ]
},
{
  "group": []
},
{
  "group": [
    "group3",
    "group4"
  ]
}

How can I filter or aggregate the documents so that I only return the last element of each group array including the blank arrays?

Expected result:

["group1", "group2", "", "group4"]

mickl · Accepted Answer · 2020-11-02 05:42:15Z

1

You can $group and use $arrayElemAt to get the last element. Additionally you need $ifNull to specify the default value (empty string):

db.collection.aggregate([
    {
        $group: {
            _id: null,
            lastElements: { $push: { $ifNull: [ { $arrayElemAt: [ "$group", -1 ] }, "" ] } }
        }
    }
])

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answered Nov 2, 2020 at 5:42

mickl

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1 Comment

louisnelza Over a year ago

Thank you @mickl! This is exactly what I needed.

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