MongoDB - $filter an array of objects with $exists

Question

I have an array of objects and in the $project pipeline of MongoDB I have to choose the one element whose metadata does not exist. So, for example below are a few documents after my $group pipeline -

{
  _id: {
      genre: "suspense",
    },
   price: 10210.6,
   data: [
      {
        subGenre: "Thriller",
        flag: true,
      },
      {
        subGenre: "jumpScare",
      },
      {
        subGenre: "horror",
        flag: true,
      }
    ]
}

After this, I need to run a $project pipeline where I have to only project that element of the data array where the flag does not exist. My syntax for that is -

db.collection.aggregate([
  {
    "$project": {
      "_id": 0,
      "price": 1,
      "data": {
        "$getField": {
          "field": "subGenre",
          "input": {
            "$first": {
             "$filter": { input: "$data", cond: { "$exists": [ "$$this.flag", false ] } }
            }
          }
        }
      }
    }
  }
])

But this is throwing an error -

Invalid $project :: caused by :: Unrecognized expression '$exists'

The output should be -

{
  price: 10210.6,
  subGenre: "jumpScare"
}

Yong Shun · Accepted Answer · 2023-01-03 07:31:29Z

2

Instead of using $exists which is invalid, you can compare the value with:

$eq: [
  "$$this.flag",
  undefined
]

This aims to check whether the flag field does not exist.

Demo @ Mongo Playground

You may also work with checking the $type value is matched with "missing". (Tested this works in MongoDB Compass)

$eq: [
  { $type: "$$this.flag" },
  "missing"
]

edited Jan 3, 2023 at 7:31

answered Jan 3, 2023 at 5:59

Yong Shun

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3 Comments

Roomi Over a year ago

This works fine here in thre mongo playground but its not returning any value in my mongodb compass and in my code also in nodejs idk why

Roomi Over a year ago

Also why is $exists invalid? Is there any other way instead of these both?

Yong Shun Over a year ago

Updated the answer. $exists is a query operator, not possible to work in the aggregation pipeline.

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