Search…
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
Last modified 1mo ago