Limit number of values in a field using MongoDB?

To limit the number of values in a field using MongoDB, use the $slice projection operator. This operator allows you to return only a specified number of elements from an array field.

Syntax

db.collection.find(
    {},
    { "arrayField": { "$slice": numberOfElements } }
);

Sample Data

Let us first create a collection with documents ?

db.numberOfValuesDemo.insertOne({
    "Values": [100, 200, 300, 900, 1000, 98]
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5cefb736ef71edecf6a1f6ab")
}

Display all documents from the collection ?

db.numberOfValuesDemo.find().pretty();

{
    "_id": ObjectId("5cefb736ef71edecf6a1f6ab"),
    "Values": [
        100,
        200,
        300,
        900,
        1000,
        98
    ]
}

Example: Limit to First 2 Values

The following query limits the Values array to return only the first 2 elements ?

db.numberOfValuesDemo.find(
    {},
    { "Values": { "$slice": 2 } }
);

{
    "_id": ObjectId("5cefb736ef71edecf6a1f6ab"),
    "Values": [100, 200]
}

Additional Examples

Last 3 Elements

db.numberOfValuesDemo.find(
    {},
    { "Values": { "$slice": -3 } }
);

{
    "_id": ObjectId("5cefb736ef71edecf6a1f6ab"),
    "Values": [900, 1000, 98]
}

Skip and Limit

Skip first 2 elements and return next 2 ?

db.numberOfValuesDemo.find(
    {},
    { "Values": { "$slice": [2, 2] } }
);

{
    "_id": ObjectId("5cefb736ef71edecf6a1f6ab"),
    "Values": [300, 900]
}

Key Points

• Positive number: Returns first N elements from the beginning
• Negative number: Returns last N elements from the end
• Array format [skip, limit]: Skips elements and returns the specified count

Conclusion

The $slice operator provides flexible array limiting capabilities in MongoDB projections. Use positive values for first elements, negative for last elements, or array syntax to skip and limit specific ranges.