Check if value exists for a field in a MongoDB document?

To check if a value exists for a field in a MongoDB document, you can use the $exists operator with the find() method. This operator checks whether a field exists and optionally whether it contains any values.

Syntax

db.collection.find({ "fieldName": { $exists: true } });
db.collection.find({ "arrayField.0": { $exists: true } });

Sample Data

db.checkIfValueDemo.insertMany([
    {
        "PlayerName": "John Smith",
        "PlayerScores": [5000, 98595858, 554343]
    },
    {
        "PlayerName": "John Doe",
        "PlayerScores": []
    },
    {
        "PlayerName": "Carol Taylor",
        "PlayerScores": [7848474, 8746345353]
    },
    {
        "PlayerName": "David Miller",
        "PlayerScores": []
    }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5cc6f507af8e7a4ca6b2ad98"),
        ObjectId("5cc6f512af8e7a4ca6b2ad99"),
        ObjectId("5cc6f521af8e7a4ca6b2ad9a"),
        ObjectId("5cc6f531af8e7a4ca6b2ad9b")
    ]
}

Example 1: Check if Array Has Values

To check if the PlayerScores array contains at least one element, query for the existence of the first array element ?

db.checkIfValueDemo.find({"PlayerScores.0": {$exists: true}});

{
    "_id": ObjectId("5cc6f507af8e7a4ca6b2ad98"),
    "PlayerName": "John Smith",
    "PlayerScores": [5000, 98595858, 554343]
}
{
    "_id": ObjectId("5cc6f521af8e7a4ca6b2ad9a"),
    "PlayerName": "Carol Taylor",
    "PlayerScores": [7848474, 8746345353]
}

Example 2: Count Documents with Non-Empty Arrays

db.checkIfValueDemo.find({"PlayerScores.0": {$exists: true}}).count();

2

Key Points

• $exists: true checks if a field exists in the document.
• "arrayField.0": {$exists: true} checks if an array has at least one element.
• Use $exists: false to find documents where a field does not exist.

Conclusion

The $exists operator effectively checks field presence and array content. Use dot notation with array indexes to verify if arrays contain values rather than just existing as empty arrays.