$last (aggregation accumulator)
On this page
Definition
$last
Returns the value that results from applying an expression to the last document in a group of documents that share the same group by a field. Only meaningful when documents are in a defined order.
$last
is only available in the$group
stage.Note
Disambiguation
The following page describes the accumulator
$last
, available only within the$group
stage. For the array operator$last
, see$last (array operator)
instead.
Syntax
$last
has the following syntax:
{ $last: <expression> }
For more information on expressions, see Expressions.
Behavior
When using $last
in a $group
stage, the output
value depends on the order of the documents coming into pipeline. To
guarantee a defined order, the $group
pipeline stage should
follow a $sort
stage.
Example
Consider a sales
collection with the following documents:
{ "_id" : 1, "item" : "abc", "date" : ISODate("2014-01-01T08:00:00Z"), "price" : 10, "quantity" : 2 } { "_id" : 2, "item" : "jkl", "date" : ISODate("2014-02-03T09:00:00Z"), "price" : 20, "quantity" : 1 } { "_id" : 3, "item" : "xyz", "date" : ISODate("2014-02-03T09:05:00Z"), "price" : 5, "quantity" : 5 } { "_id" : 4, "item" : "abc", "date" : ISODate("2014-02-15T08:00:00Z"), "price" : 10, "quantity" : 10 } { "_id" : 5, "item" : "xyz", "date" : ISODate("2014-02-15T09:05:00Z"), "price" : 5, "quantity" : 10 } { "_id" : 6, "item" : "xyz", "date" : ISODate("2014-02-15T12:05:10Z"), "price" : 5, "quantity" : 5 } { "_id" : 7, "item" : "xyz", "date" : ISODate("2014-02-15T14:12:12Z"), "price" : 5, "quantity" : 10 }
The following operation first sorts the documents by item
and
date
, and then in the following $group
stage, groups
the now sorted documents by the item
field and uses the
$last
accumulator to compute the last sales date for each item:
db.sales.aggregate( [ { $sort: { item: 1, date: 1 } }, { $group: { _id: "$item", lastSalesDate: { $last: "$date" } } } ] )
The operation returns the following results:
{ "_id" : "xyz", "lastSalesDate" : ISODate("2014-02-15T14:12:12Z") } { "_id" : "jkl", "lastSalesDate" : ISODate("2014-02-03T09:00:00Z") } { "_id" : "abc", "lastSalesDate" : ISODate("2014-02-15T08:00:00Z") }