MongoDB extract same field from each object in Array

Question

I am performing an aggregation query and I end up with the following Array that contains many objects.

How can I extract the field _id from each one of the afore-mentioned Objects?

Expected output would be an array of each of the _ids inside each object

Expected output would be an array of each of the "_id"s inside each object — azal
– azal, Commented Aug 1, 2021 at 15:20
Hi @azal can you share the actual data and query so that we can reproduce it — HandsomeCoder
– HandsomeCoder, Commented Aug 1, 2021 at 15:26
Using $project and $map: example. Is this example correct for you? — J.F.
– J.F., Commented Aug 1, 2021 at 15:27

J.F. · Accepted Answer · 2021-08-01 15:33:48Z

4

To get an array of every _id into you object you can use a $map.

Map is similar as JS map, check this:

const array = [
  {"id": 1,"other": ""},
  {"id": 2,"other": ""},
  {"id": 3,"other": ""},
  {"id": 4,"other": ""},
  {"id": 5,"other": ""}]
console.log(array.map(m => m.id))

You can get this approach using mongo $map into a $project stage like this:

With this aggregation you are creating a field called array where using $map return array.id, so it creates an array of desired ids.

db.collection.aggregate([
  {
    "$project": {
      "_id": 0,
      "array": {
        "$map": {
          "input": "$array",
          "as": "a",
          "in": "$$a.id"
        }
      }
    }
  }
])

Example here

answered Aug 1, 2021 at 15:33

J.F.

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2 Comments

azal Over a year ago

That was exactly what I was looking for! I was quite confident about the usage of the map function but did not know how to use it. Cheers

J.F. Over a year ago

No problem! Also if the issue has been solved, mark answer as accepted is appreciated.

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