$first and $last accumulators in MongoDB - Step-by-Step Execution

Concept Flow - $first and $last accumulators

Start with documents

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Sort documents if needed

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Group documents by key

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$first picks first document's field

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$last picks last document's field

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Output grouped results with $first and $last values

Documents are sorted if needed, then grouped by a key. $first and $last pick the first and last values from each group after sorting.

Execution Sample

MongoDB

db.sales.aggregate([
  {$sort: {date: 1}},
  {$group: {
    _id: "$store",
    firstSale: {$first: "$amount"},
    lastSale: {$last: "$amount"}
  }}
])

This query sorts sales by date ascending, groups by store, then picks the first and last sale amounts per store.

Execution Table

Step	Input Document	Group Key (_id)	Sorted Order	$first Value	$last Value	Action
1	{store: 'A', date: '2023-01-01', amount: 100}	A	1	100	100	First document for group A, first and last both 100
2	{store: 'A', date: '2023-01-05', amount: 150}	A	2	100	150	Second document for A, last updated to 150
3	{store: 'B', date: '2023-01-02', amount: 200}	B	1	200	200	First document for B, first and last both 200
4	{store: 'B', date: '2023-01-10', amount: 250}	B	2	200	250	Second document for B, last updated to 250
5	Aggregation complete	-	-	100	150	Group A final first and last values
6	Aggregation complete	-	-	200	250	Group B final first and last values

💡 All documents processed, groups output with $first and $last values

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
Group A $first	null	100	100	100	100	100
Group A $last	null	100	150	150	150	150
Group B $first	null	null	null	200	200	200
Group B $last	null	null	null	200	250	250

Key Moments - 2 Insights

Why does $first not change after the first document in a group?

$first always takes the value from the first document in the sorted group, so after the first document, it remains unchanged as shown in rows 1 and 2 of the execution_table.

How does sorting affect $first and $last values?

Sorting determines the order of documents in each group. $first picks the first document after sorting, and $last picks the last. Without sorting, the order is unpredictable.

Visual Quiz - 3 Questions

Look at the execution_table at Step 2, what is the $last value for group A?

A 100

B 150

C null

D 200

💡 Hint

Check the $last Value column in row 2 of the execution_table.

At which step does group B first get a $first value assigned?

A Step 1

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the Group Key and $first Value columns in the execution_table.

If the documents were not sorted by date, how would $first and $last behave?

A They would pick values from documents in an unpredictable order.

B They would still pick the earliest and latest dates correctly.

C They would cause an error.

D They would pick the same value for both $first and $last.

💡 Hint

Refer to the key_moments about sorting's effect on $first and $last.

Concept Snapshot

$first and $last accumulators in MongoDB aggregation:
- Use inside $group stage
- $first returns the first value in the group (after sorting)
- $last returns the last value in the group (after sorting)
- Sorting documents before grouping controls which values are first/last
- Useful to get earliest/latest values per group

Full Transcript

This visual execution trace shows how MongoDB's $first and $last accumulators work inside an aggregation pipeline. Documents are first sorted by date, then grouped by store. For each group, $first picks the amount from the earliest date document, and $last picks the amount from the latest date document. The execution table tracks each document processed, the group it belongs to, and how $first and $last values update. The variable tracker shows these values step-by-step. Key moments clarify why $first stays the same after the first document and how sorting affects results. The quiz tests understanding of these steps and the importance of sorting. This helps beginners see how $first and $last pick values from grouped documents in order.