$last (aggregation)
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Definition
Syntax
$last syntax:
{ $last: <expression> }
Behaviors
Array Operator
If the expression resolves to an array:
For a group of documents, as with the
$groupstage,$lastreturns the entire array from the last document. It does not traverse array elements.For an individual document, as with the
$addFieldsstage,$lastreturns the last element of the array.
Missing Values
Documents in a group may be missing fields or may have fields with missing values.
If there are no documents from the prior pipeline stage, the
$groupstage returns nothing.If the field that the
$lastaccumulator is processing is missing,$lastreturnsnull.
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") }