The MongoDB Aggregation Framework

The MongoDB Aggregation Framework Chapter 1 - Basic Aggregation - $match and $project

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$match: Filtering documents

Which of the following is/are true of the $match stage?

$match can only filter documents on one field.
It uses the familiar MongoDB query language.
It should come very early in an aggregation pipeline.
$match can use both query operators and aggregation expressions.

Shaping documents with $project

Which of the following statements are true of the $project stage?

$project can only be used once within an Aggregation pipeline.
$project cannot be used to assign new values to existing fields.
Beyond simply removing and retaining fields, $project lets us add new fields.
Once we specify a field to retain or perform some computation in a $project stage, we must specify all fields we wish to retain. The only exception to this is the _id field.

Optional Lab - Expressions with $project

Let's find how many movies in our movies collection are a "labor of love", where the same person appears in cast, directors, and writers

Note that you may have a dataset that has duplicate entries for some films. Don't worry if you count them few times, meaning you should not try to find those duplicates.

To get a count after you have defined your pipeline, there are two simple methods.

// add the $count stage to the end of your pipeline

// you will learn about this stage shortly!

db.movies.aggregate([

{$stage1},

{$stage2},

...$stageN,

{ $count: "labors of love" }

])

// or use itcount()

db.movies.aggregate([

{$stage1},