How to apply a condition only if field exists in MongoDB?

To apply a condition only if a field exists in MongoDB, use the $or operator combined with $exists. This allows you to match documents where a field either doesn't exist or meets a specific condition.

Syntax

db.collection.find({
    $or: [
        { fieldName: { $exists: false } },
        { fieldName: condition }
    ]
});

Sample Data

Let's create a collection with some documents where the StudentAge field is missing in one document ?

db.applyConditionDemo.insertMany([
    { "StudentName": "Larry", "StudentAge": 21, "StudentMarks": 45 },
    { "StudentName": "Sam", "StudentAge": 23, "StudentMarks": 55 },
    { "StudentName": "David", "StudentAge": 21, "StudentMarks": 65 },
    { "StudentName": "Carol", "StudentAge": 24, "StudentMarks": 78 },
    { "StudentName": "Chris", "StudentAge": 21, "StudentMarks": 88 },
    { "StudentName": "Robert", "StudentMarks": 98 }
]);

Display all documents to see the structure ?

db.applyConditionDemo.find().pretty();

{
    "_id" : ObjectId("5cb80b78623186894665ae36"),
    "StudentName" : "Larry",
    "StudentAge" : 21,
    "StudentMarks" : 45
}
{
    "_id" : ObjectId("5cb80b87623186894665ae37"),
    "StudentName" : "Sam",
    "StudentAge" : 23,
    "StudentMarks" : 55
}
{
    "_id" : ObjectId("5cb80b95623186894665ae38"),
    "StudentName" : "David",
    "StudentAge" : 21,
    "StudentMarks" : 65
}
{
    "_id" : ObjectId("5cb80ba3623186894665ae39"),
    "StudentName" : "Carol",
    "StudentAge" : 24,
    "StudentMarks" : 78
}
{
    "_id" : ObjectId("5cb80bae623186894665ae3a"),
    "StudentName" : "Chris",
    "StudentAge" : 21,
    "StudentMarks" : 88
}
{
    "_id" : ObjectId("5cb80c3d623186894665ae3b"),
    "StudentName" : "Robert",
    "StudentMarks" : 98
}

Example: Apply Condition Only If Field Exists

Find all documents where StudentAge is 21 OR the field doesn't exist at all ?

db.applyConditionDemo.find({
    $or: [
        { StudentAge: { $exists: false } },
        { StudentAge: 21 }
    ]
}).pretty();

{
    "_id" : ObjectId("5cb80b78623186894665ae36"),
    "StudentName" : "Larry",
    "StudentAge" : 21,
    "StudentMarks" : 45
}
{
    "_id" : ObjectId("5cb80b95623186894665ae38"),
    "StudentName" : "David",
    "StudentAge" : 21,
    "StudentMarks" : 65
}
{
    "_id" : ObjectId("5cb80bae623186894665ae3a"),
    "StudentName" : "Chris",
    "StudentAge" : 21,
    "StudentMarks" : 88
}
{
    "_id" : ObjectId("5cb80c3d623186894665ae3b"),
    "StudentName" : "Robert",
    "StudentMarks" : 98
}

How It Works

• { StudentAge: { $exists: false } } matches documents where StudentAge field is not present
• { StudentAge: 21 } matches documents where StudentAge equals 21
• $or combines both conditions, returning documents that satisfy either condition

Conclusion

Use $or with $exists: false to apply conditions only when fields exist. This technique helps handle documents with inconsistent schema structures effectively.