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 stage. $addToSet has the following syntax: { $addToSet: <expression> } For more information on expressions, see . BehaviorIf 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. ExampleConsider 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" ] } ← $push (aggregation) $stdDevPop (aggregation) → © MongoDB, Inc 2008-2017. MongoDB, Mongo, and the leaf logo are registered trademarks of MongoDB, Inc. |