$group (aggregation) — MongoDB Manual

The $group stage separates documents into groups according to a
“group key”. The output is one document for each unique group key.

A group key is often a field, or group of fields. The group key can
also be the result of an expression. Use the _id field in the
$group pipeline stage to set the group key. See below for
usage examples .

In the $group stage output, the _id field is set to the
group key for that document.

The output documents can also contain additional fields that are
set using accumulator expressions .

Note

$group does not order its output documents.

The $group stage has the following prototype form:

{  
$group
:     {      





_id
: < expression > ,         < field1 > :  { < accumulator1 >  : < expression1 >  } ,       ...    } }

Field

Description

_id

_id expression specifies the group key.
If you specify an _id value of null, or any other
constant value, the $group stage returns a single
document that aggregates values across all of the input
documents.

Required. Theexpression specifies the group key. If you specify anvalue of null, or any other constant value, thestage returns a single document that aggregates values across all of the input documents. See the Group by Null example .

field

Optional. Computed using the accumulator operators .

The _id and the accumulator operators
can accept any valid expression. For more information on
expressions, see Expressions .