I am running an aggregate function below. A document may or may not have an element, and I just want to return true/false. The element is huge if it does exist, so returning the entire element creates many problems and is not needed.

To preface the problem, I am in a production environment on version 3.0.4 and upgrading to 3.4 is not an option at this time, although it looks like that version has better solutions.

To test this, I have a document in a collection mycollection. The document has an element exists which is an object containing other elements. It does not have an element called notexists

db.runCommand({
    "aggregate": "mycollection",
    "pipeline": [{
        "$match": {...}
    }, {
        "$sort": {...}
    }, {
        "$group": {...}
    }, {
        "$limit": 10
    }, {
        "$project": {
            "aggregated": {
                "$map": {
                    "input": "$mycollection",
                    "as": "document",
                    "in": {
                        "exists_test":{"$eq":["$$document.exists",null]},
                        "not_exists_test":{"$eq":["$$document.notexists",null]},
                        "exists_test_ifnull":{"$ifNull":["$$document.exists","test"]},
                        "not_exists_test_ifnull":{"$ifNull":["$$document.notexists","test"]},
                        "exists_content": "$$document.exists"
                        ...
                    }
                }
            },
            "success": {
                "$cond": {
                    "if": { "$gt": ["$status", 0] },
                    "then": "false",
                    "else": "true"
                }
            }
        }
    }]
})

Unfortunately, this returns a document:

{
    aggregated: [{
        "exists_test": false, (correct)
        "not_exists_test": false, (wrong)
        "exists_test_ifnull": content from document.exists, (correct)
        "not_exists_test_ifnull": "test", (correct)
        "exists_content": content from document.exists, (correct)
    }]
}

It seems that "not_exists_test":{"$eq":["$$document.notexists",null]}, should return true, since $ifNull does accurately reflect that the value is null.