Aggregation stages

Question 1

$addFields

Answer 1

Adds new fields to documents. Similar to $project, $addFields reshapes each document in the stream; specifically, by adding new fields to output documents that contain both the existing fields from the input documents and the newly added fields.

$set is an alias for $addFields.

Question 2

$bucket

Answer 2

Categorizes incoming documents into groups, called buckets, based on a specified expression and bucket boundaries.

Question 3

$bucketAuto

Answer 3

Categorizes incoming documents into a specific number of groups, called buckets, based on a specified expression. Bucket boundaries are automatically determined in an attempt to evenly distribute the documents into the specified number of buckets.

Question 4

$changeStream

Answer 4

Returns a Change Stream cursor for the collection. This stage can only occur once in an aggregation pipeline and it must occur as the first stage.

Question 5

$collStats

Answer 5

Returns statistics regarding a collection or view.

Question 6

$count

Answer 6

Returns a count of the number of documents at this stage of the aggregation pipeline.

Distinct from the $count aggregation accumulator.

Question 7

$currentOp

Answer 7

Returns information about current operations in MongoDB.

Question 8

$densify

Answer 8

Creates new documents in a sequence of documents where certain values in a field are missing.

Question 9

$documents

Answer 9

Returns literal documents from input expressions.

Question 10

$facet

Answer 10

Processes multiple aggregation pipelines within a single stage on the same set of input documents. Enables the creation of multi-faceted aggregations capable of characterizing data across multiple dimensions, or facets, in a single stage.

Question 11

$fill

Answer 11

Populates null and missing field values within documents.

Question 12

$geoNear

Answer 12

Returns an ordered stream of documents based on the proximity to a geospatial point. Incorporates the functionality of $match, $sort, and $limit for geospatial data. The output documents include an additional distance field and can include a location identifier field.

Question 13

$graphLookup

Answer 13

Performs a recursive search on a collection. To each output document, adds a new array field that contains the traversal results of the recursive search for that document.

Question 14

$group

Answer 14

Groups input documents by a specified identifier expression and applies the accumulator expression(s), if specified, to each group. Consumes all input documents and outputs one document per each distinct group. The output documents only contain the identifier field and, if specified, accumulated fields.

Question 15

$indexStats

Answer 15

Returns statistics regarding the use of each index for the collection.

Question 16

$limit

Answer 16

Passes the first n documents unmodified to the pipeline where n is the specified limit. For each input document, outputs either one document (for the first n documents) or zero documents (after the first n documents).

Question 17

$listLocalSessions

Answer 17

Lists the sessions in the current transaction.

Question 18

$listSessions

Answer 18

Lists all sessions that have been active long enough to propagate to the system.sessions collection.

Question 19

$lookup

Answer 19

Performs a left outer join to another collection in the same database to filter in documents from the “joined” collection for processing.

Question 20

$match

Answer 20

Filters the document stream to allow only matching documents to pass unmodified into the next pipeline stage. $match uses standard MongoDB queries. For each input document, outputs either one document (a match) or zero documents (no match).

Question 21

$merge

Answer 21

Writes the resulting documents of the aggregation pipeline to a collection. The stage can incorporate (insert new documents, merge documents, replace documents, keep existing documents, fail the operation, process documents with a custom update pipeline) the results into an output collection. To use the $merge stage, it must be the last stage in the pipeline.

New in version 4.2.

Question 22

$out

Answer 22

Writes the resulting documents of the aggregation pipeline to a collection. To use the $out stage, it must be the last stage in the pipeline.

Question 23

$planCacheStats

Answer 23

Returns plan cache information for a collection.

Question 24

$project

Answer 24

Reshapes each document in the stream, such as by adding new fields or removing existing fields. For each input document, outputs one document.

See also $unset for removing existing fields

Question 25

$redact

Answer 25

Reshapes each document in the stream by restricting the content for each document based on information stored in the documents themselves. Incorporates the functionality of $project and $match. Can be used to implement field level redaction. For each input document, outputs either one or zero documents.

Question 26

$replaceRoot

Answer 26

Replaces a document with the specified embedded document. The operation replaces all existing fields in the input document, including the _id field. Specify a document embedded in the input document to promote the embedded document to the top level.

$replaceWith is an alias for $replaceRoot stage.

Question 27

$replaceWith

Answer 27

Replaces a document with the specified embedded document. The operation replaces all existing fields in the input document, including the _id field. Specify a document embedded in the input document to promote the embedded document to the top level.

$replaceWith is an alias for $replaceRoot stage.

Question 28

$sample

Answer 28

Randomly selects the specified number of documents from its input.

Question 29

$set

Answer 29

Adds new fields to documents. Similar to $project, $set reshapes each document in the stream; specifically, by adding new fields to output documents that contain both the existing fields from the input documents and the newly added fields.

$set is an alias for $addFields stage.

Question 30

$setWindowFields

Answer 30

Groups documents into windows and applies one or more operators to the documents in each window.

New in version 5.0.

Question 31

$shardedDataDistribution

Answer 31

Provides statistics on the distribution of data across shards.

Question 32

$skip

Answer 32

Skips the first n documents where n is the specified skip number and passes the remaining documents unmodified to the pipeline. For each input document, outputs either zero documents (for the first n documents) or one document (if after the first n documents).

Question 33

$sort

Answer 33

Reorders the document stream by a specified sort key. Only the order changes; the documents remain unmodified. For each input document, outputs one document.

Question 34

$sortByCount

Answer 34

Groups incoming documents based on the value of a specified expression, then computes the count of documents in each distinct group.

Question 35

$unionWith

Answer 35

Performs a union of two collections; i.e. combines pipeline results from two collections into a single result set.

New in version 4.4.

Question 36

$unset

Answer 36

Removes/excludes fields from documents.

$unset is an alias for $project stage that removes fields.

Question 37

$unwind

Answer 37

Deconstructs an array field from the input documents to output a document for each element. Each output document replaces the array with an element value. For each input document, outputs n documents where n is the number of array elements and can be zero for an empty array.