Accumulators

Group Accumulators

These functions are available for use in the $group stage, accumulators are operators that maintain their state (e.g. totals, maximums, minimums, and related data) as information progress through the pipeline.

When used as accumulators in the $group stage, these operators take as input a single expression, evaluating the expression once for each input document, and maintain their stage for the group of documents that share the same group key.

For example:

Variable	Access via $	Brief Description
accumulator	$accumulator	Returns the result of a user-defined accumulator function
addToSet	$addToSet	Returns an array of unique expression values for each group. Order of the array elements is undefined
avg	$avg	Returns an average of numerical values. Ignores non-numeric values

Accumulators

Some operators that are available as accumulators for the $group stage are also available for use in other. When used in these other stages, these operators do not maintain their state and can take as input either a single argument or multiple arguments.

For example:

Variable	Access via $	Brief Description
addToSet	$addtoSet	Returns array of unique pushed values
avg	$avg	Calculates the average of a number property or expression in the group.
count	$count	Performs a count of a grouped documents or, if the property is an array, a count of the number of elements.
first	$first	Returns the first value from the group. The result depends on the sort field(s)
last	$last	Returns the last value from the group. The result depends on the sort field(s)
mergeObjects	$mergeObjects	Combines multiple documents into a single document
min	$min	Returns the smallest value from the group.
max	$max	Returns the largest value from the group
push	$push	Returns the array of pushed values
sum	$sum	Calculates the sum of a number property or expression in the group