MongoDB Compass: Filter query on array object is not working

December 6, 2021

I want to filter the array object based on value provided.
I am using following query in Filter text box and along with projection. Seems like there filter is not getting applied.

I also tried following but unfortunately array is not getting filtered

  { MissingPersonIds : {$elemMatch: { PhotoId : '2 - Copy (3).jpg'}} }

Filter:

{ "MissingPersonIds.PhotoId" : "2 - Copy (3).jpg" }

Projection:

{ MissingPersonIds : { $slice: [1,10] }}

Schema is:

[{
  "_id": {
    "$oid": "61ada7da9a30fd8471869bbc"
  },
  "ImportKeyId": 5843,
  "MissingPersonIds": [
    {
      "PhotoId": "2 - Copy.jpg",
      "Description": "Account ID not found"
    },
    {
      "PhotoId": "2 - Copy (2).jpg",
      "Description": "Account ID not found"
    },
    {
      "PhotoId": "2 - Copy (3).jpg",
      "Description": "Account ID not found"
    },
    {
      "PhotoId": "2 - Copy - Copy.jpg",
      "Description": "Account ID not found"
    },
    {
      "PhotoId": "2 - Copy - Copy (2).jpg",
      "Description": "Account ID not found"
    }
  ]
}]

The output I am expecting is :

[{
  "_id": {
    "$oid": "61ada7da9a30fd8471869bbc"
  },
  "ImportKeyId": 5843,
  "MissingPersonIds": [
    {
      "PhotoId": "2 - Copy (3).jpg",
      "Description": "Account ID not found"
    }
  ]
}]

>Solution :

You need $filter in the projection to filter element(s) in an array.

db.collection.find({},
{
  "MissingPersonIds": {
    $filter: {
      input: "$MissingPersonIds",
      cond: {
        "$eq": [
          "$$this.PhotoId",
          "2 - Copy (3).jpg"
        ]
      }
    }
  }
})

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