$addToSet (aggregation)

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.

$addToSet is only available in the $group stage.

$addToSet has the following syntax:

{ $addToSet: <expression> }

For more information on expressions, see Expressions.

Behavior

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; i.e. the existing document has the exact same fields and values in the exact same order.

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" ] }