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$last (aggregation accumulator)

$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.

$last has the following syntax:

{ $last: <expression> }

For more information on expressions, see Expressions.

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.

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") }
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  • Definition
  • Syntax
  • Behavior
  • Example