$last (aggregation)

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Definition

Syntax
Behaviors
Example

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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.

Syntax

$last syntax:

{ $last: <expression> }

Behaviors

Array Operator

If the expression resolves to an array:

For a group of documents, as with the $group stage, $last returns the entire array from the last document. It does not traverse array elements.
For an individual document, as with the $addFields stage, $last returns 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 $group stage returns nothing.
If the field that the $last accumulator is processing is missing, $last returns null.

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") }

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$isoWeekYear (aggregation)

$last (array operator)