How to convert array of documents to array in mongodb aggregation

Question

I have a document similar to this:

{
  "field1": "a",
  "field2": {
    "subfield1": "sa",
    "subfield2": [
      {
        "name": "abc"
      },
      {
        "name": "def"
      }
    ]
  }
}

I want something like this:

{
   "field1":"a",
   "field2":{
      "subfield1":"sa",
      "subfield2":["abc","def"]
   }
}

Thanks in advance!

Does this answer your question? Create array of strings from array of documents in MongoDB — Nate Anderson
– Nate Anderson, Commented Dec 7, 2023 at 4:45

J.F. · Accepted Answer · 2021-11-12 09:09:35Z

2

Using projection into find:

db.collection.find({/*your find query*/},
{
  "field1": 1,
  "field2.subfield1": 1,
  "field2.subfield2": "$field2.subfield2.name"
})

Example here

Using aggregate:

db.collection.aggregate([
  {
    "$project": {
      "field1": 1,
      "field2.subfield1": 1,
      "field2.subfield2": "$field2.subfield2.name"
    }
  }
])

Example here

Also you can add "_id": 0, to not output the _id. Examples here and here

edited Nov 12, 2021 at 9:09

answered Nov 11, 2021 at 22:07

J.F.

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3 Comments

sadsun Over a year ago

But this doesn't give me output like this "subfield2":["abc","def"] instead gives me nested arrays and documents

J.F. Over a year ago

Oops sorry, I forgot "field2.subfield1": 1,. Now is fixed. The output is the same as desired one: "subfield2":["abc","def"]

Nate Anderson Over a year ago

Like @sadsun, the first of these doesn't work for me (using db.collection.find with a projection argument) (even though I see it works in your mongoplayground.net link) it only works when I use db.collection.aggregate with a $project stage. I am on MongoDB version 3.6.6. Notice both these other answers use aggregate, here and here

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