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$first (aggregation accumulator)

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$first

Changed in version 5.0.

Returns the value that results from applying an expression to the first document in a group of documents. Only meaningful when documents are in a defined order.

$first is available in these stages:

Note
Disambiguation

This page describes the $first aggregation accumulator. For the $first array operator, see $first (array operator).

$first syntax:

{ $first: <expression> }

For more information on expressions, see Expressions.

To define the document order for $first with the:

Note

Although the $sort stage passes ordered documents as input to the $group and $setWindowFields stages, those stages are not guaranteed to maintain the sort order in their own output.

When used with $setWindowFields, $first returns null for empty windows. An example empty window is a { documents: [ -1, -1 ] } documents window on the first document of a partition.

Consider a sales collection with the following documents:

{ "_id" : 1, "item" : "abc", "price" : 10, "quantity" : 2, "date" : ISODate("2014-01-01T08:00:00Z") }
{ "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1, "date" : ISODate("2014-02-03T09:00:00Z") }
{ "_id" : 3, "item" : "xyz", "price" : 5, "quantity" : 5, "date" : ISODate("2014-02-03T09:05:00Z") }
{ "_id" : 4, "item" : "abc", "price" : 10, "quantity" : 10, "date" : ISODate("2014-02-15T08:00:00Z") }
{ "_id" : 5, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-02-15T09:05:00Z") }
{ "_id" : 6, "item" : "xyz", "price" : 5, "quantity" : 5, "date" : ISODate("2014-02-15T12:05:10Z") }
{ "_id" : 7, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-02-15T14:12:12Z") }

Grouping the documents by the item field, the following operation uses the $first accumulator to compute the first sales date for each item:

db.sales.aggregate(
[
{ $sort: { item: 1, date: 1 } },
{
$group:
{
_id: "$item",
firstSalesDate: { $first: "$date" }
}
}
]
)

The operation returns the following results:

{ "_id" : "xyz", "firstSalesDate" : ISODate("2014-02-03T09:05:00Z") }
{ "_id" : "jkl", "firstSalesDate" : ISODate("2014-02-03T09:00:00Z") }
{ "_id" : "abc", "firstSalesDate" : ISODate("2014-01-01T08:00:00Z") }

New in version 5.0.

Create a cakeSales collection that contains cake sales in the states of California (CA) and Washington (WA):

db.cakeSales.insertMany( [
{ _id: 0, type: "chocolate", orderDate: new Date("2020-05-18T14:10:30Z"),
state: "CA", price: 13, quantity: 120 },
{ _id: 1, type: "chocolate", orderDate: new Date("2021-03-20T11:30:05Z"),
state: "WA", price: 14, quantity: 140 },
{ _id: 2, type: "vanilla", orderDate: new Date("2021-01-11T06:31:15Z"),
state: "CA", price: 12, quantity: 145 },
{ _id: 3, type: "vanilla", orderDate: new Date("2020-02-08T13:13:23Z"),
state: "WA", price: 13, quantity: 104 },
{ _id: 4, type: "strawberry", orderDate: new Date("2019-05-18T16:09:01Z"),
state: "CA", price: 41, quantity: 162 },
{ _id: 5, type: "strawberry", orderDate: new Date("2019-01-08T06:12:03Z"),
state: "WA", price: 43, quantity: 134 }
] )

This example uses $first in the $setWindowFields stage to output the first cake sales order type for each state:

db.cakeSales.aggregate( [
{
$setWindowFields: {
partitionBy: "$state",
sortBy: { orderDate: 1 },
output: {
firstOrderTypeForState: {
$first: "$type",
window: {
documents: [ "unbounded", "current" ]
}
}
}
}
}
] )

In the example:

  • partitionBy: "$state" partitions the documents in the collection by state. There are partitions for CA and WA.
  • sortBy: { orderDate: 1 } sorts the documents in each partition by orderDate in ascending order (1), so the earliest orderDate is first.
  • output sets the firstOrderTypeForState field to the first order type from the documents window.

    The window contains documents between an unbounded lower limit and the current document in the output. This means $first returns the first order type for the documents between the beginning of the partition and the current document.

In this output, the first order type value for CA and WA is shown in the firstOrderTypeForState field:

{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
"state" : "CA", "price" : 41, "quantity" : 162, "firstOrderTypeForState" : "strawberry" }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
"state" : "CA", "price" : 13, "quantity" : 120, "firstOrderTypeForState" : "strawberry" }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
"state" : "CA", "price" : 12, "quantity" : 145, "firstOrderTypeForState" : "strawberry" }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
"state" : "WA", "price" : 43, "quantity" : 134, "firstOrderTypeForState" : "strawberry" }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
"state" : "WA", "price" : 13, "quantity" : 104, "firstOrderTypeForState" : "strawberry" }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
"state" : "WA", "price" : 14, "quantity" : 140, "firstOrderTypeForState" : "strawberry" }
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On this page

  • Definition
  • Syntax
  • Behavior
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