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$addToSet (aggregation)![](/docs/v5.3/assets/link.svg)
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Definition![](/docs/v5.3/assets/link.svg)
Changed in version 5.0.
$addToSet
returns an array of all unique values that
results from applying an expression
to each document in a group.
The order of the elements in the returned array is unspecified.
$addToSet
is available in these stages:
$setWindowFields
(Available starting in MongoDB 5.0)
Syntax![](/docs/v5.3/assets/link.svg)
$addToSet
syntax:
{ $addToSet: <expression> }
For more information on expressions, see Expressions.
Behavior![](/docs/v5.3/assets/link.svg)
If the value of the expression is an array, $addToSet
appends
the whole array as a single element.
If the value of the expression is a document, MongoDB determines that the document is a duplicate if another document in the array matches the to-be-added document exactly. Specifically, the existing document has the exact same fields and values in the exact same order.
Examples![](/docs/v5.3/assets/link.svg)
Use in $group
Stage![](/docs/v5.3/assets/link.svg)
Consider a sales
collection with the following documents:
{ "_id" : 1, "item" : "abc", "price" : 10, "quantity" : 2, "date" : ISODate("2014-01-01T08:00:00Z") } { "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1, "date" : ISODate("2014-02-03T09:00:00Z") } { "_id" : 3, "item" : "xyz", "price" : 5, "quantity" : 5, "date" : ISODate("2014-02-03T09:05:00Z") } { "_id" : 4, "item" : "abc", "price" : 10, "quantity" : 10, "date" : ISODate("2014-02-15T08:00:00Z") } { "_id" : 5, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-02-15T09:12:00Z") }
Grouping the documents by the day and the year of the date
field,
the following operation uses the $addToSet
accumulator to
compute the list of unique items sold for each group:
db.sales.aggregate( [ { $group: { _id: { day: { $dayOfYear: "$date"}, year: { $year: "$date" } }, itemsSold: { $addToSet: "$item" } } } ] )
The operation returns the following results:
{ "_id" : { "day" : 46, "year" : 2014 }, "itemsSold" : [ "xyz", "abc" ] } { "_id" : { "day" : 34, "year" : 2014 }, "itemsSold" : [ "xyz", "jkl" ] } { "_id" : { "day" : 1, "year" : 2014 }, "itemsSold" : [ "abc" ] }
Use in $setWindowFields
Stage![](/docs/v5.3/assets/link.svg)
New in version 5.0.
Create a cakeSales
collection that contains cake sales in the states
of California (CA
) and Washington (WA
):
db.cakeSales.insertMany( [ { _id: 0, type: "chocolate", orderDate: new Date("2020-05-18T14:10:30Z"), state: "CA", price: 13, quantity: 120 }, { _id: 1, type: "chocolate", orderDate: new Date("2021-03-20T11:30:05Z"), state: "WA", price: 14, quantity: 140 }, { _id: 2, type: "vanilla", orderDate: new Date("2021-01-11T06:31:15Z"), state: "CA", price: 12, quantity: 145 }, { _id: 3, type: "vanilla", orderDate: new Date("2020-02-08T13:13:23Z"), state: "WA", price: 13, quantity: 104 }, { _id: 4, type: "strawberry", orderDate: new Date("2019-05-18T16:09:01Z"), state: "CA", price: 41, quantity: 162 }, { _id: 5, type: "strawberry", orderDate: new Date("2019-01-08T06:12:03Z"), state: "WA", price: 43, quantity: 134 } ] )
This example uses $addToSet
in the $setWindowFields
stage to output the unique cake type
sales for each state
:
db.cakeSales.aggregate( [ { $setWindowFields: { partitionBy: "$state", sortBy: { orderDate: 1 }, output: { cakeTypesForState: { $addToSet: "$type", window: { documents: [ "unbounded", "current" ] } } } } } ] )
In the example:
partitionBy: "$state"
partitions the documents in the collection bystate
. There are partitions forCA
andWA
.sortBy: { orderDate: 1 }
sorts the documents in each partition byorderDate
in ascending order (1
), so the earliestorderDate
is first.
output
adds each unique caketype
to thecakeTypesForState
array field using$addToSet
that is run in a documents window.The window contains documents between an
unbounded
lower limit and thecurrent
document. This means$addToSet
returns an array containing the unique caketype
fields for the documents between the beginning of the partition and the current document.
In this example output, the cake type
array for CA
and WA
is
shown in the cakeTypesForState
field:
{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"), "state" : "CA", "price" : 41, "quantity" : 162, "cakeTypesForState" : [ "strawberry" ] } { "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"), "state" : "CA", "price" : 13, "quantity" : 120, "cakeTypesForState" : [ "strawberry", "chocolate" ] } { "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"), "state" : "CA", "price" : 12, "quantity" : 145, "cakeTypesForState" : [ "strawberry", "vanilla", "chocolate" ] } { "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"), "state" : "WA", "price" : 43, "quantity" : 134, "cakeTypesForState" : [ "strawberry" ] } { "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"), "state" : "WA", "price" : 13, "quantity" : 104, "cakeTypesForState" : [ "vanilla", "strawberry" ] } { "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"), "state" : "WA", "price" : 14, "quantity" : 140, "cakeTypesForState" : [ "vanilla", "chocolate", "strawberry" ] }