$first and $last accumulators in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $first accumulator do in MongoDB aggregation?

The $first accumulator returns the first value from the documents in each group, based on the order they are processed.

beginner

What is the purpose of the $last accumulator in MongoDB aggregation?

The $last accumulator returns the last value from the documents in each group, based on the order they are processed.

intermediate

How does the order of documents affect the results of $first and $last?

The order of documents determines which values $first and $last pick. If documents are not sorted, the results may be unpredictable.

intermediate

Can $first and $last be used without sorting documents first?

Yes, but the values returned depend on the natural order of documents, which may not be consistent. Sorting first ensures predictable results.

beginner

Give a simple example of using $first and $last in a MongoDB aggregation pipeline.

Example: Group sales by product and get the first and last sale dates:

{ $group: { _id: "$product", firstSale: { $first: "$date" }, lastSale: { $last: "$date" } } }

What does the $first accumulator return in a MongoDB aggregation?

A The total count of documents in the group

B The last value in the group based on document order

C The first value in the group based on document order

D The average of all values in the group

$first returns the first value in the group according to the order documents are processed.

Which accumulator returns the last value in each group in MongoDB aggregation?

A $last

B $first

C $max

D $min

$last returns the last value in the group based on document order.

Why is sorting documents important before using $first or $last?

A To reduce memory usage

B To make the results predictable

C To speed up the query

D Sorting is not important

Sorting ensures $first and $last pick values from a known order, making results predictable.

If documents are not sorted, what can happen when using $first and $last?

A They return values based on natural document order, which may be unpredictable

B They return random values

C They always return null

D They cause an error

Without sorting, $first and $last use the natural order, which can vary and be unpredictable.

Which MongoDB stage is typically used before $group to ensure correct $first and $last results?

A $match

B $limit

C $project

D $sort

$sort orders documents so $first and $last return expected values.

Explain how $first and $last accumulators work in MongoDB aggregation and why document order matters.

Think about how MongoDB processes documents in a group.

Describe a scenario where using $first and $last would be useful in a real-world application.

Consider tracking changes over time or events.