$addToSet (aggregation)
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Definition
$addToSet-
Returns an array of all unique values that results from applying an expression to each document in a group of documents that share the same group by key. Order of the elements in the output array is unspecified.
$addToSetis only available in the$groupstage.$addToSethas the following syntax:{ $addToSet: <expression> }For more information on expressions, see Expressions.
Behavior
Array Expression
If the value of the expression is an array, $addToSet appends the whole array as a single element.
Document Expression
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; i.e. the existing document has the exact same fields and values in the exact same order.
Memory Restrictions
Starting in 3.6.17, $addToSet has a memory limit of 100 MiB (100 * 1024 * 1024), even when db.collection.aggregate() is run with allowDiskUse: true.
See aggregration pipeline limitations for more information.
Example
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" ] }