MongoDB extract value from array of objects into simple array

Question

I have a collection of documents that I initially set up like this:

{
  _id: "some_id",
  data: [
    {
      _id: "_id_foo",
      value: "foo"
    },
    {
      _id: "_id_bar",
      value: "bar"
    }
  ]
}

Now, I realized I don't need the value field, so I can turn the array of objects into an array of strings ["_id_foo", "_id_bar"]. I'm trying to build a pipeline to accomplish this, but I'm failing miserably.

If it's easier, I can also create a data_new array and delete the old one later.

Thanks!

Just use $map for this

nimrod serok
– nimrod serok

2022-10-20 09:03:27 +00:00
Commented Oct 20, 2022 at 9:03 — nimrod serok
– nimrod serok, Commented Oct 20, 2022 at 9:03

SuleymanSah · Accepted Answer · 2022-10-20 09:22:54Z

2

You can use the map operator and convert the array of objects to array of strings.

db.collection.aggregate([
  {
    $addFields: {
      data: {
        $map: {
          input: "$data",
          as: "data",
          in: "$$data._id"
        }
      }
    }
  }
])

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answered Oct 20, 2022 at 9:22

SuleymanSah

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2 Comments

Andrea Di Stefano Over a year ago

Thanks, that did the trick! I didn't know about map

user20042973 Over a year ago

If the goal is to persist this change permanently, then consider using this aggregation pipeline as the second argument to the update command

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