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$first (aggregation)
Definition
-
$first
Returns the value that results from applying an expression to the
first document in a group of documents that share the same group by
key. Only meaningful when documents are in a defined order.
$first
is only available in the
$group
stage.
$first
has the following syntax:
For more information on expressions, see Expressions.
Behavior
When using $first
in a $group
stage, the
$group
stage should follow a $sort
stage to
have the input documents in a defined order.
Note
Although the $sort
stage passes ordered documents as
input to the $group
stage, $group
is not
guaranteed to maintain this sort order in its own output.
Example
Consider a sales
collection with the following documents:
Grouping the documents by the item
field, the following operation
uses the $first
accumulator to compute the first sales date for
each item:
The operation returns the following results: