group

$group

Separates documents into groups according to a group key you define and outputs one document for each unique group key. You can also use accumalator operators to compute data for each group.

A group key can be an existing field, a group of fields, or the result of an expression. Use the _id field in the $group pipeline stage to set the group key.

Note: The $group method does not order its output documents - instead, you can sort documents with a $sort stage earlier in the pipeline.

Syntax

{
  "$group":
    {
      "_id": "<expression>", // Group key
      "<customField1>": { "<accumulator1>" : "<expression1>" },
    }
 }

Field	Type	Description	Required
_id	expression	Define an expression for the group key.	Required
Custom field		Define a custom field, then define an accumulator operator and expression pair to compute the data you want for the custom field. You can define multiple custom fields.	Optional

Accumulator operators

Accumulator operator	Returns	Notes
$addToSet	An array of unique expression values for each group	Order of the array elements is undefined
$avg	An average of numerical values	Ignores non-numeric values
$count	The number of documents in a group
$first	A value from the first document for each group	For order, sort documents with a $sort stage earlier in the pipeline.
$last	A value from the last document for each group	For order, sort documents with a $sort stage earlier in the pipeline.
$max	The highest expression value for each group
$min	The lowest expression value for each group
$push	An array of expression values for documents in each group
$stdDevPop	The population standard deviation of the input values
$sum	A sum of numerical values	Ignores non-numeric values

Example

In the following example, in the $group stage of the pipeline, the group key is defined by telemetry sensor source id. The custom fields are defined as follows:

• "count": Counts the documents for each group
• "avgtemp": Computes the average of the temp values for each group
• "low": Gets the temp value of the first document in the group using the $first operator. The documents are sorted by temp in ascending order in a previous stage.
• "high": Gets the temp value of the last document in the group using the $last operator. The documents are sorted by temp in ascending order in a previous stage.

Sample Request

[
	{
		"$match": {
			"temp": {
				"$exists": true
			},
			"_tsMetadata._sourceId": {
				"$exists": true
			}
		}
	},
	{
		"$sort": {
			"temp": 1
		}
	},
	{
		"$group": {
			"_id": "$_tsMetadata._sourceId",
			"count": {
				"$count": {}
			},
			"avgtemp": {
				"$avg": "$temp"
			},
			"low": {
				"$first": "$temp"
			},
			"high": {
				"$last": "$temp"
			}
		}
	}
]

Sample Response

{
	"_list": [
		{
			"high": 63,
			"low": 63,
			"count": 1,
			"_id": "b1da8f7b-d6fd-46e0-a7f1-69d2118c05c2",
			"avgtemp": 63
		},
		{
			"high": 65,
			"low": 38,
			"count": 3,
			"_id": "7d4a1c16-c2e4-4159-a5b8-5814f4bfe6e2",
			"avgtemp": 55.333333333333336
		},
		{
			"high": 83,
			"low": 43,
			"count": 3,
			"_id": "ef4b5a5d-a5dd-48b0-b9c1-3618552cfe09",
			"avgtemp": 61.333333333333336
		},
		{
			"high": 85,
			"low": 52,
			"count": 3,
			"_id": "ed11cc38-97c8-486e-b21e-2cccb7271173",
			"avgtemp": 72.33333333333333
		}
	]
}