$last (aggregation)
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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.$last
has the following syntax:For more information on expressions, see Expressions.
Behavior
When using $last
in a $group
stage, the $group
stage should follow a $sort
stage to have the input documents in a defined order.
Example
Consider a sales
collection with the following documents:
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:
The operation returns the following results: