👐 Sorting

We can sort our results using the $sort stage. We need to sort on a field, with 1 being ascending and -1 descending order.

👐 Let's get all books from 1984, sorted by number of pages. We'll only show the title, pages, and authors.

Atlas UI

[
    {$match: {year: 1984}},
    {$project: {_id: 0, title: 1, pages: 1, authors: 1}},
    {$sort: {"pages": 1}},
]

MongoDB Shell

let booksFrom1984 = {$match: {year: 1984}};
let showOnlyTheseFields = {$project: {_id: 0, title: 1, pages: 1, authors: 1}};
let slimmerBooksFirst = {$sort: {"pages": 1}}

db.books.aggregate([
    booksFrom1984,
    showOnlyTheseFields,
    slimmerBooksFirst,
]);

As we can see, books with no pages info appear first. These documents don't have that information. Hence, pages is null and gets sorted as less than any number.

👐 We can sort the other way around:

Atlas UI

[
    {$match: {year: 1984}},
    {$project: {_id: 0, title: 1, pages: 1, authors: 1}},
    {$sort: {"pages": -1}},
]

MongoDB Shell

let booksFrom1984 = {$match: {year: 1984}};
let showOnlyTheseFields = {$project: {_id: 0, title: 1, pages: 1, authors: 1}};
let thickerBooksFirst = {$sort: {"pages": -1}}

db.books.aggregate([
    booksFrom1984,
    showOnlyTheseFields,
    thickerBooksFirst,
]);

👐 How can we get the thickest book from 1984?

Answer

Atlas UI

[
    {$match: {year: 1984}},
    {$project: {_id: 0, title: 1, pages: 1, authors: 1}},
    {$sort: {"pages": -1}},
    {$limit: 1}
]

MongoDB Shell

let booksFrom1984 = {$match: {year: 1984}};
let showOnlyTheseFields = {$project: {_id: 0, title: 1, pages: 1, authors: 1}};
let thickerBooksFirst = {$sort: {"pages": -1}}
let justOne = {$limit: 1}

db.books.aggregate([
    booksFrom1984,
    showOnlyTheseFields,
    thickerBooksFirst,
    justOne
]);