Aggregation Native

Question 1

What is an aggregation pipeline?

Answer 1

An aggregation pipeline is a set of aggregation stages (operations) executed on documents to produce a result

Question 2

What is the general syntax for aggregation pipelinies?

Answer 2

db.collection.aggregate([{pipeline1}, {pipeline2}], {options}) returns cursor

Question 3

Do aggregation pipelines modify documents in their respective collections?

Answer 3

No, unless a $merge or $out is used

Question 4

True or False

When you run aggregation pipelines on MongoDB Atlas deployments in the MongoDB Atlas UI, you can preview the results at each stage

Answer 4

True

Question 5

What are the 7 most common aggregation stages?

Answer 5

1. $match
2. $group
3. $sort
4. $set
5. $project
6. $out
7. $merge

Question 6

Can the same aggregation stages be used multiple times in the same pipeline?

Answer 6

Yes, except $merge and $out

Question 7

What is the purpose of field paths in aggregation pipelines?

Answer 7

The purpose of field paths in aggregation pipelines is to provide access to the field values of input documents. Syntax: "$fieldName"

Question 8

Code an example of the aggregation pipeline structure

Answer 8

db.collection.aggregate([{$stage1: {...}}, {$stage2: {...}}])

Question 9

What is the purpose of $match?

Answer 9

The purpose of $match is to filter documents in an aggregation pipeline

Question 10

What is the purpose of $group?

Answer 10

The purpose of $group is to separate documents into groups according to a group key. This is normally done to perform accumulator operations

Question 11

Code an example of a $group stage

Answer 11

db.collection.aggregate([{$group: {_id: "$groupKey", field: {$accumulatorOperator: expression}}}])

Question 12

Code an example using the $sort and $limit stages

Answer 12

db.collection.aggregate([{$sort: {field: 1}}, {$limit: 3}])

Question 13

What is the purpose of the $project stage?

Answer 13

The purpose of the $project stage is to select the fields that should be displayed in output documents. It can also be used to set or add new fields

Question 14

Code an example of the $count stage

Answer 14

db.collection.aggregate([{$match: ...}, {$count: fieldNameOfOutputDocument}])

Question 15

What is the purpose of the $out stage?

Answer 15

The purpose of the $out stage is to insert the resulting documents of an aggregation pipeline into a new collection. However, if an existing collection is selected, all of the documents in that collection will be replaced

Question 16

True or False

The $out stage must be the last stage in an aggregation pipeline

Answer 16

True

Question 17

Code an example using the $project stage

Answer 17

db.collection.aggregate([{$project: {field: 1 or 0}}])

Question 18

What are the 2 main stages that can benefit from indexes?

Answer 18

1. $match
2. $sort

Question 19

What does aggregate() return?

Answer 19

A cursor

Question 20

True or False

During the pipeline processing, the documents may exceed the 16 MB size

Answer 20

True. However, care must be taken to ensure when the documents are returned to the client, they do not exceed the 16 MB limit

Question 21

How many aggregation pipeline stages are allowed in an aggregation pipeline?

Answer 21

1000 stages

Question 22

What is the output of a $group stage?

Answer 22

The output of a $group stage is one document for each group key

Question 23

Code an example of using a $lookup stage and explain how it works

Answer 23

db.collection.aggregate({$lookup: {from: "otherCollection", localField: "fieldName", foreignField: "otherField", as: "fieldArrayName"}})

If the local field value matches the foreign field value, the foreign document will be appended to the field array name

Question 24

What are the 2 general structures of the $out stage?

Answer 24

1 db.collection.aggregate([{$out: "collectionName"}])

2 db.collection.aggregate([{$out: {db: "databaseName", coll: "collectionName"}}])

Question 25

# True or False The `$out` stage does not change any indexes that existed on the previous collection

Answer 25

True

Question 26

# True or False If the `$out` operation modifies a collection with an Atlas Search index, you must delete and re-create the search index

Answer 26

True

Question 27

Which 3 stages cannot appear multiple times in an aggregation pipeline?

Answer 27

1. `$out` 2. `$merge` 3. `$geoNear`

Question 28

# True or False An aggregation pipeline can use indexes from the input collection to improve performance.

Answer 28

True