Multi-key Indexing on an entire array with MongoDB?

Multi-key indexing in MongoDB allows you to index entire arrays and their nested elements efficiently. When you create an index on an array field, MongoDB automatically creates a multi-key index that indexes each array element separately.

Syntax

db.collection.createIndex({ "arrayField": 1 })
db.collection.find({ "arrayField": { "nestedField": "value" } })

Create Sample Data

db.demo277.insertMany([
    { "details": [{ "FirstName": "John" }] },
    { "details": [{ "FirstName": "David" }] },
    { "details": [{ "FirstName": "Chris" }] }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e48fb21dd099650a5401a52"),
        ObjectId("5e48fb27dd099650a5401a53"),
        ObjectId("5e48fb2bdd099650a5401a54")
    ]
}

Verify Sample Data

db.demo277.find();

{ "_id": ObjectId("5e48fb21dd099650a5401a52"), "details": [{ "FirstName": "John" }] }
{ "_id": ObjectId("5e48fb27dd099650a5401a53"), "details": [{ "FirstName": "David" }] }
{ "_id": ObjectId("5e48fb2bdd099650a5401a54"), "details": [{ "FirstName": "Chris" }] }

Create Multi-key Index

db.demo277.createIndex({ "details": 1 });

Query Array Elements

Query to find documents where the details array contains an object with FirstName "David" ?

db.demo277.find({ "details": { "FirstName": "David" } });

{ "_id": ObjectId("5e48fb27dd099650a5401a53"), "details": [{ "FirstName": "David" }] }

How Multi-key Indexing Works

• MongoDB automatically detects array fields and creates multi-key indexes
• Each array element gets indexed separately for efficient queries
• Queries can match exact array elements or use dot notation for nested fields

Conclusion

Multi-key indexing enables efficient querying of array elements in MongoDB. Create an index on the array field, and MongoDB automatically indexes each element for fast lookups and filtering operations.