MongoDB Aggregation Framework Flashcards

Which aggregation expression operator concatenates strings?

Answer: $concat

$concat takes an array of strings or string expressions and concatenates them into a single string.

What is the purpose of the $facet stage in MongoDB aggregation?

Answer: Processes multiple aggregation pipelines on the same input documents

$facet enables multi-dimensional grouping by running several sub-pipelines on the same set of input documents within one stage.

Which operator in $group keeps the first value of a field encountered per group?

Answer: $first

$first returns the value of the field from the first document in each group as documents pass through the pipeline.

What does the $bucket stage do in an aggregation pipeline?

Answer: Groups documents into defined ranges based on a specified expression

$bucket categorizes incoming documents into groups (buckets) based on a specified expression and defined boundaries.

Which aggregation stage skips over a specified number of documents?

Answer: $skip

$skip passes over the specified number of documents, outputting only those that come after the skipped count.

What does the allowDiskUse option do in the aggregate() method?

Answer: Permits aggregation stages to use temporary files when memory limit is exceeded

allowDiskUse: true allows aggregation stages to write temporary files to disk when the 100MB memory limit per stage is exceeded.