mongoDB projection on array in an object

Question

I have this document structure in the collection:

{"_id":"890138075223711744",
"guildID":"854557773990854707",
"name":"test-lab",
"game": {
    "usedWords":["akşam","elma","akım"]
    }
}

What is the most efficient way to get its fields except the array (it can be really large), and at the same time, see if an item exists in the array ?

I tried this:

    let query = {_id: channelID}
    const options = { sort: { name: 1 }, projection: { name: 1, "game.usedWords": { $elemMatch: { word}}}}
    mongoClient.db(db).collection("channels").findOne(query, options);

but I got the error: "$elemMatch can not be used on nested fields"

J.F. · Accepted Answer · 2022-02-21 01:10:22Z

1

If I've understood correctly you can use this query:

Using positional operator $ you can return only the matched word.

db.collection.find({
  "game.usedWords": "akşam"
},
{
  "name": 1,
  "game.usedWords.$": 1
})

Example here

The output is only name and the matched word (also _id which is returned by default)

[
  {
    "_id": "890138075223711744",
    "game": {
      "usedWords": [
        "akşam"
      ]
    },
    "name": "test-lab"
  }
]

answered Feb 21, 2022 at 1:10

J.F.

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3 Comments

Tolga Gülsuna Over a year ago

It works ok but if the array does not include the item, the query returns null. I want other fields like "name" to be returned regardless.

J.F. Over a year ago

Is something like this what are you looking for?

Tolga Gülsuna Over a year ago

this solves the problem. Credits to Centrax from mongoDB discord server. @J.F.

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