How to find if element exists in document - MongoDB?

To find if an element exists in a MongoDB document, use the $exists operator. This operator checks whether a specified field is present in the document, regardless of its value.

Syntax

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

Sample Data

db.demo497.insertMany([
    {"details": [{"Name": "Chris"}, {"Name": "Bob"}]},
    {"details": [{"Name": "Carol"}]},
    {"details": [{}]}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e84b3cfb0f3fa88e22790d1"),
        ObjectId("5e84b3d9b0f3fa88e22790d2"),
        ObjectId("5e84b3e9b0f3fa88e22790d3")
    ]
}

Display all documents from the collection ?

db.demo497.find();

{ "_id": ObjectId("5e84b3cfb0f3fa88e22790d1"), "details": [{"Name": "Chris"}, {"Name": "Bob"}] }
{ "_id": ObjectId("5e84b3d9b0f3fa88e22790d2"), "details": [{"Name": "Carol"}] }
{ "_id": ObjectId("5e84b3e9b0f3fa88e22790d3"), "details": [{}] }

Example: Check if Name Field Exists

Find documents where the Name field exists within the details array ?

db.demo497.find({"details.Name": {$exists: true}});

{ "_id": ObjectId("5e84b3cfb0f3fa88e22790d1"), "details": [{"Name": "Chris"}, {"Name": "Bob"}] }
{ "_id": ObjectId("5e84b3d9b0f3fa88e22790d2"), "details": [{"Name": "Carol"}] }

Example: Using $or with $exists

Find documents where either the Name field is not "Carol" or the details field doesn't exist ?

db.demo497.find({
    $or: [
        {"details.Name": {$ne: "Carol"}},
        {"details": {$exists: false}}
    ]
});

{ "_id": ObjectId("5e84b3cfb0f3fa88e22790d1"), "details": [{"Name": "Chris"}, {"Name": "Bob"}] }
{ "_id": ObjectId("5e84b3e9b0f3fa88e22790d3"), "details": [{}] }

Key Points

• $exists: true returns documents where the field is present
• $exists: false returns documents where the field is absent
• Works with nested fields using dot notation

Conclusion

The $exists operator is essential for checking field presence in MongoDB documents. Combine it with $or, $ne, and other operators for complex existence-based queries.