Aggregation task - $unwind

A field producing task that returns a duplicated document for each value in the specified array.

The $unwind task produces a duplicated document for each value in an array, such that in the resulting document the array field object is replaced by that value.

Syntax diagram

Command arguments

fieldname: The name of an array field. The name must be preceded with $.

Example

Consider a set of documents that contain a name, year (array), and city:

john, [2002], washington
jane, [2002, 2005], philadelphia

db.article.aggregate(
 { $project : { name : 1, year : 1, city: 1 } },
 { $unwind : "$year" }
)

Sample output:

john, 2002, washington
jane, 2002, philadelphia
jane, 2005, philadelphia