Just like arrays, we also want to search inside hashes. Searching inside hashes involves keys and values. Let's assume that the book object looks as follows (that is, a hash instead of an array):

{
categories: {
'drama': 1,
'thriller': 2
},
}

We can search for all books that have the drama set as 1:

> db.books.find({ "categories.drama" : 1 })

Notice that we access hash fields just like standard JSON object access.