$first, $last, and $top/$bottom Accumulators
Learners will pick the first or last document per group and use the newer $top and $bottom accumulators to pick by a sort key.
$first, $last, and $top/$bottom Accumulators is a free MongoDB Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the MongoDB Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Picking One Document Per Group
Sometimes aggregation needs to select a single representative document from each group rather than computing a numeric aggregate. MongoDB provides several accumulators for this: $first, $last, and the newer $top and $bottom accumulators. Each has different semantics for determining which document is selected, and choosing the right one affects both correctness and performance.
$first: Taking the Earliest Document Value
$first returns the value from the first document encountered in the group. Since MongoDB does not guarantee document processing order within a $group unless you sort first, $first is meaningful only when you place a $sort stage immediately before the $group. This two-stage pattern—sort then group—is the standard way to reliably pick the earliest or highest-priority item per group.
db.orders.aggregate([
// Sort by date ascending so $first gets the oldest order
{ $sort: { createdAt: 1 } },
{
$group: {
_id: '$customerId',
firstOrderDate: { $first: '$createdAt' },
firstOrderAmount: { $first: '$amount' },
firstOrderId: { $first: '$_id' }
}
}
])$last: Taking the Latest Document Value
$last returns the value from the last document encountered in the group. Again, this is meaningful only when combined with a preceding $sort. Sort ascending and use $last to get the most recent value per group. Sort descending and use $first to achieve the same result. The most common use case is finding each user's or customer's most recent activity.
db.loginEvents.aggregate([
// Sort ascending: $last will be the most recent event
{ $sort: { timestamp: 1 } },
{
$group: {
_id: '$userId',
lastLoginTime: { $last: '$timestamp' },
lastLoginIp: { $last: '$ipAddress' },
totalLogins: { $sum: 1 }
}
}
])Performance Cost of Sort Before Group
The pattern of $sort before $group has an important performance implication: it prevents MongoDB from pushing the $group in front of the $sort during pipeline optimization. If the collection is large and you have an index on the sort field, MongoDB can use it for the sort phase but still must sort all documents before grouping. For very large collections, this can be expensive. The $top and $bottom accumulators were introduced partly to address this limitation.
// Potentially expensive on large collections:
// MongoDB must sort ALL documents before grouping
db.events.aggregate([
{ $sort: { timestamp: -1 } }, // sorts entire collection
{
$group: {
_id: '$userId',
latestEvent: { $first: '$eventType' }
}
}
])$top: Pick by Sort Key Within Group
Introduced in MongoDB 5.2, $top lets you pick the top document within each group according to a sort specification—without a separate $sort stage. You specify a sortBy expression and an output expression directly inside the accumulator. MongoDB evaluates the sort per group, which can be more efficient than sorting the entire collection first.
db.orders.aggregate([
{
$group: {
_id: '$customerId',
mostRecentOrder: {
$top: {
sortBy: { createdAt: -1 }, // highest date first
output: {
orderId: '$_id',
amount: '$amount',
date: '$createdAt'
}
}
}
}
}
])$bottom: Pick the Last by Sort Key
$bottom is the complement to $top—it returns the document that would appear last after sorting by the given key within each group. Use $bottom with sortBy: { timestamp: 1 } to get the earliest event per group, or use $top with the opposite sort direction—both approaches are equivalent. Choose whichever expresses your intent more clearly.
db.gameScores.aggregate([
{
$group: {
_id: '$playerId',
highestScore: {
$top: {
sortBy: { score: -1 },
output: { score: '$score', level: '$level', date: '$date' }
}
},
lowestScore: {
$bottom: {
sortBy: { score: -1 },
output: { score: '$score', level: '$level', date: '$date' }
}
}
}
}
])$topN and $bottomN: Picking Multiple Items
MongoDB also provides $topN and $bottomN accumulators that return an array of the top or bottom N documents per group according to a sort key. This is useful for leaderboard-style queries where you want the top 3 scores per level or the 5 most recent orders per customer, all computed in a single $group stage.
db.scores.aggregate([
{
$group: {
_id: '$gameId',
top3Players: {
$topN: {
n: 3,
sortBy: { score: -1 },
output: { playerId: '$playerId', score: '$score' }
}
}
}
}
])Choosing Between $first/$last and $top/$bottom
Use $first/$last when you are already sorting the pipeline for other reasons and want a simple, familiar API. Use $top/$bottom when you only need the selection behavior and want to avoid the cost of a full collection sort before $group. In practice, $top/$bottom with appropriate indexes can be significantly faster for large collections because MongoDB may be able to optimize the per-group selection more efficiently.
// Prefer $top when no prior $sort is needed for other stages:
db.orders.aggregate([
{
$group: {
_id: '$region',
latestOrder: {
$top: { sortBy: { date: -1 }, output: { amount: '$amount', date: '$date' } }
}
}
}
]);
// Use $first when you are already sorting for other accumulators:
db.orders.aggregate([
{ $sort: { date: -1 } },
{
$group: {
_id: '$region',
latestDate: { $first: '$date' },
latestAmount: { $first: '$amount' },
allAmounts: { $push: '$amount' } // also needs sorted input
}
}
])Practical Use Case: Latest Price Per Product
A classic real-world use case is maintaining a price history collection and querying the latest effective price for each product. Using $top with sort by date descending makes this query concise and efficient. The result is one document per product containing its current price without needing a separate 'current price' collection.
db.priceHistory.aggregate([
{
$group: {
_id: '$productId',
currentPrice: {
$top: {
sortBy: { effectiveDate: -1 },
output: {
price: '$price',
currency: '$currency',
effectiveDate: '$effectiveDate'
}
}
}
}
}
])Combining Accumulators in One $group
You can freely combine $first, $last, $top, $bottom, $sum, $avg, and $push in a single $group stage. Each accumulator operates independently on the documents in the group. This lets you compute summary statistics alongside picking representative documents in one efficient aggregation stage, avoiding multiple passes over the data.
db.transactions.aggregate([
{ $sort: { timestamp: 1 } },
{
$group: {
_id: '$accountId',
firstTransaction: { $first: '$timestamp' },
lastTransaction: { $last: '$timestamp' },
transactionCount: { $sum: 1 },
totalVolume: { $sum: '$amount' },
avgTransaction: { $avg: '$amount' },
largestTransaction: { $max: '$amount' }
}
}
])Version Compatibility Notes
$first and $last have been available since early MongoDB versions. The $top and $bottom accumulators were introduced in MongoDB 5.2, and $topN/$bottomN also became available around that time. If you are using an older MongoDB version (pre-5.2), fall back to the $sort then $first/$last pattern. Check your Atlas cluster version or self-hosted version before using newer operators.
Quick Check
Test your understanding of MongoDB & NoSQL Databases concepts from this lesson.
Lesson Recap
In this lesson you learned: $first/$last pick values from the first/last document in a group (requires prior $sort), $top/$bottom (MongoDB 5.2+) pick by sort key within the group without a separate $sort stage, and $topN/$bottomN return arrays of the top or bottom N items per group. Next up we explore window functions with $setWindowFields for running totals and moving averages.
Frequently asked questions
Is the “$first, $last, and $top/$bottom Accumulators” lesson free?
Yes — the full text of “$first, $last, and $top/$bottom Accumulators” is free to read here on the web, and the MongoDB Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the MongoDB Academy course, upgrade to CoddyKit PRO.
What will I learn in “$first, $last, and $top/$bottom Accumulators”?
Learners will pick the first or last document per group and use the newer $top and $bottom accumulators to pick by a sort key. You practise MongoDB Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start MongoDB Academy?
No prior experience is required. MongoDB Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “$first, $last, and $top/$bottom Accumulators” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this MongoDB Academy lesson?
Yes. Every MongoDB Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- $sum, $avg, $min, $max: Numeric Aggregation
- $push and $addToSet: Building Arrays in Groups
- $first, $last, and $top/$bottom Accumulators
- Window Functions With $setWindowFields