At this point, we should be familiar with querying in MongoDB using the .find() method. Let’s take things a step further by learning how we can use this same method to filter documents based on array fields.

Consider a collection called books where each document shares a similar structure to the following:

{
  _id: ObjectId(...),
  title: "Alice in Wonderland",
  author: "Lewis Carroll",
  year_published: 1865,
  genres: ["childrens", "fantasy", "literary nonsense"]
}

Imagine we are looking for a new book to dive into – specifically, one that spans the young adult, fantasy, and adventure genres. We can query the collection for an array containing these exact values by using the .find() method and passing in a query argument that includes the field and array we want to match:

db.books.find({ genres: ["young adult", "fantasy", "adventure"] })

This query would return documents where the genres field is an array containing exactly three values, "young adult", "fantasy", and "adventure". For example, we would get a result that might look like this:

{
  _id: ObjectId(...),
  title: "Harry Potter and The Sorcerer's Stone",
  author: "JK Rowling",
  year_published: 1997,
  genres: ["young adult", "fantasy", "adventure"]
},
{
  _id: ObjectId(...),
  title: "The Gilded Ones",
  author: "Namina Forna",
  year_published: 2021,
  genres: ["young adult", "fantasy", "adventure"]
}

Note that this query would only return documents where the array field contains precisely the values included in the query in the specified order. The following document contains the same values as mentioned in our query, but it wouldn’t be matched by our search because these values are in a different order:

{
  _id: ObjectId(...),
  title: "Children of Blood and Bone",
  author: "Tomi Adeyemi",
  year_published: 2018,
  genres: ["fantasy", "young adult", "adventure"]
}

The following document would also not be matched because it contains an additional value not specified by our query:

{
  _id: ObjectId(...),
  title: "Eragon",
  author: "Christopher Paolini,
  year_published: 2002,
  genres: ["young adult", "fantasy", "adventure", "science fiction"]
}

Before moving on, let’s practice querying array fields for exact matches!