How to filtering list with count sub array's field?

Question

This is example one of document in MongoDB.

{
    "content" : "I just said that before. I'll never be with you.",
    "analysis" : [
        {
            "sentence" : "I just said that before.",
            "sentiment" : "neutral",
            "subject" : "I",
            "expression" : "say"
        },
        {
            "sentence" : "I'll never be with you.",
            "sentiment" : "negative",
            "subject" : "I",
            "expression" : "be",
            "label": "Farewell"
        }
    ]
}

There every analysis element can has "label" or no.

I want to filter this document for put "label" in analysis element.

I need document list that analysis.label.count < analysis.count

I tried this query
{ $and: [{ "analysis": { $exists: true } }, { "analysis.label": { $exists: false } }] }.

But it returns only has no label in any analysis' element.

Do I need aggregate()? or find() with query?

@turivishal chaged query { $and: [{ "analysis": { $exists: true } }, { "analysis.label": null }] } It work! I don't know how 'null' works. — 01hanst
– 01hanst, Commented Nov 5, 2021 at 8:33

turivishal · Accepted Answer · 2021-11-05 08:46:38Z

1

It requires $elemMatch operator to match $exists false condition in array,

{
  analysis: {
    $exists: true,
    $elemMatch: {
      label: {
        $exists: false
      }
    }
  }
}

Playground

The second approach checking null condition,

{
  analysis: {
    $exists: true
  },
  "analysis.label": null
}

Playground

edited Nov 5, 2021 at 8:46

answered Nov 5, 2021 at 8:37

turivishal

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2 Comments

01hanst Over a year ago

Oh, It work. Is there difference with your condition and this { $and: [{ "analysis": { $exists: true } }, { "analysis.label": null }] }? Both query returns same result.

turivishal Over a year ago

the $exists will check if the property exists in the document, and the null will check both the conditions, if the field exists or if it exists then null? so both will work in your situation.

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