MongoDB Aggregation Framework Flashcards

Which aggregation stage performs a left outer join to another collection?

Answer: $lookup

$lookup performs a left outer join with another collection in the same database, adding matched documents as an array field.

What does the $avg accumulator compute in a $group stage?

Answer: The arithmetic mean of numeric values

$avg calculates the average of all numeric values for documents in each group.

Which stage sorts the documents in the aggregation pipeline?

Answer: $sort

$sort reorders pipeline documents according to specified fields, using 1 for ascending and -1 for descending.

What is the $addFields stage used for in aggregation?

Answer: Adds new fields to documents without removing existing ones

$addFields outputs documents containing all existing fields plus any newly added or modified fields.

Which aggregation stage writes the results of the pipeline to a specified collection?

Answer: $out

$out writes the resulting documents to a specified collection, replacing it if it already exists.

What does the $count stage return in an aggregation pipeline?

Answer: A single document with the count of documents passed to it

$count outputs a single document containing a field with the count of documents that entered the stage.