How do MongoDB indexes handle fields whose values are arrays? Conveniently, MongoDB automatically creates what’s known as a multikey index whenever an index on a array field is created. Multikey indexes provide an index key for each element in the indexed array.

Suppose we had a document within the students collection that had a field, sports with an array as its value:

{ 
  _id: ObjectId(...),
  last_name: "Tapia",
  first_name: "Joseph",
  major: "architecture",
  birth_year: 1988,
  graduation_year: 2012 ,
  sports: ["rowing", "boxing"]
}

We could create a multikey index on this field in the same way we would create any other single-field index:

db.students.createIndex({ sports : 1 });

This would create an index that references the sports field for every document in the collection. Since sports is an array field, the resulting multikey index would contain individual references to each element in the array. We specified ascending order for our index so the values would be organized in alphabetical order.

Note that this example discusses multikey single field indexes. Next we’ll learn about some important considerations to keep in mind when creating compound multikey indexes.

Let’s spend some time practicing creating and using multikey indexes!