OPTIONS

$unwind (aggregation)

$unwind

Peels off the elements of an array individually, and returns a stream of documents. $unwind returns one document for every member of the unwound array within every source document. Take the following aggregation command:

db.article.aggregate(
    { $project : {
        author : 1 ,
        title : 1 ,
        tags : 1
    }},
    { $unwind : "$tags" }
);

Note

The dollar sign (i.e. $) must precede the field specification handed to the $unwind operator.

In the above aggregation $project selects (inclusively) the author, title, and tags fields, as well as the _id field implicitly. Then the pipeline passes the results of the projection to the $unwind operator, which will unwind the tags field. This operation may return a sequence of documents that resemble the following for a collection that contains one document holding a tags field with an array of 3 items.

{
     "result" : [
             {
                     "_id" : ObjectId("4e6e4ef557b77501a49233f6"),
                     "title" : "this is my title",
                     "author" : "bob",
                     "tags" : "fun"
             },
             {
                     "_id" : ObjectId("4e6e4ef557b77501a49233f6"),
                     "title" : "this is my title",
                     "author" : "bob",
                     "tags" : "good"
             },
             {
                     "_id" : ObjectId("4e6e4ef557b77501a49233f6"),
                     "title" : "this is my title",
                     "author" : "bob",
                     "tags" : "fun"
             }
     ],
     "OK" : 1
}

A single document becomes 3 documents: each document is identical except for the value of the tags field. Each value of tags is one of the values in the original “tags” array.

Note

$unwind has the following behaviors:

  • $unwind is most useful in combination with $group.
  • You may undo the effects of unwind operation with the $group pipeline operator.
  • If you specify a target field for $unwind that does not exist in an input document, the pipeline ignores the input document, and will generate no result documents.
  • If you specify a target field for $unwind that is not an array, db.collection.aggregate() generates an error.
  • If you specify a target field for $unwind that holds an empty array ([]) in an input document, the pipeline ignores the input document, and will not generate any result documents.
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