👐 $count

How can we be sure that this pipeline is returning exactly 15 books?

Atlas UI
MongoDB Shell

[
    {$match: {$and: [{year: 1985}, {pages: {$gte: 150}}]}},
    {$project: {_id: 0, title: 1, year: 1, totalInventory: 1, available: 1}},
    {$limit: 15}
]

let moreThan150pages =  {pages: {$gte: 150}}
let booksFrom1985WithMoreThan150pages = {$match: {$and: [{year: 1985}, moreThan150pages]}};
let showOnlyTheseFields = {$project: {_id: 0, title: 1, year: 1, totalInventory: 1, available: 1}};
let getJust15books =  {$limit: 15};

db.books.aggregate([
    booksFrom1985WithMoreThan150pages,
    showOnlyTheseFields,
    getJust15books,
]);

By counting the books! Let's add a new stage to the pipeline to count those books:

Atlas UI
MongoDB Shell

[
    {$match: {$and: [{year: 1985}, {pages: {$gte: 150}} ]}},
    {$project: {_id: 0, title: 1, year: 1, totalInventory: 1, available: 1}},
    {$limit: 15},
    {$count: "totalBooks"},
]

let moreThan150pages =  {pages: {$gte: 150}}
let booksFrom1985WithMoreThan150pages = {$match: {$and: [{year: 1985}, moreThan150pages]}};
let showOnlyTheseFields = {$project: {_id: 0, title: 1, year: 1, totalInventory: 1, available: 1}};
let getJust15books =  {$limit: 15};
let count = {$count: "totalBooks"}

db.books.aggregate([
    booksFrom1985WithMoreThan150pages,
    showOnlyTheseFields,
    getJust15books,
    count,
]);

Here, $count will count the resulting docs and will return a document with just one field: totalBooks.

{
  totalBooks: 15
}

👐 How many authors do we have in our authors collection?

Answer

Atlas UI
MongoDB Shell

[ {$count: "authorCount"} ]

db.authors.aggregate([{$count: "authorCount"}])

The order of stages is important

If we count after the $match stage, the document we're passing to the next steps contains only the totalBooks field. So the $project stage will return an empty document.

Atlas UI
MongoDB Shell

[
    {$match: {$and: [{year: 1985}, {pages: {$gte: 150}} ]}},
    {$count: "totalBooks"},
    {$project: {_id: 0, title: 1, year: 1, totalInventory: 1, available: 1}},
    {$limit: 15},
]

let moreThan150pages =  {pages: {$gte: 150}}
let booksFrom1985WithMoreThan150pages = {$match: {$and: [{year: 1985}, moreThan150pages]}};
let showOnlyTheseFields = {$project: {_id: 0, title: 1, year: 1, totalInventory: 1, available: 1}};
let getJust15books =  {$limit: 15};
let count = {$count: "totalBooks"}

db.books.aggregate([
    booksFrom1985WithMoreThan150pages,
    count,
    showOnlyTheseFields,
    getJust15books,
]);

The order of stages is important​

The order of stages is important