Indexing scalar (single) values was explained in the preceding sections. However, one of the advantages that we get from using MongoDB is the ability to easily store vector values in the form of arrays.
In the relational world, storing arrays is generally frowned upon, as it violates the normal forms. In a document-oriented database such as MongoDB, it is frequently a part of our design, as we can store and query easily on complex structures of data.
Indexing arrays of documents is achieved by using the multikey index. A multikey index can store both arrays of scalar values and arrays of nested documents.
Creating a multikey index is the same as creating a regular index:
> db.books.createIndex({"tags":1})
Assume that we ...
