$unwind (aggregation)

Definition

$unwind

Deconstructs an array field from the input documents to output a document for each element. Each output document is the input document with the value of the array field replaced by the element.

The $unwind stage has one of two syntaxes:

  • The operand is a field path:

    { $unwind: <field path> }
    

    To specify a field path, prefix the field name with a dollar sign $ and enclose in quotes.

  • The operand is a document:

    New in version 3.2.

    {
      $unwind:
        {
          path: <field path>,
          includeArrayIndex: <string>,
          preserveNullAndEmptyArrays: <boolean>
        }
    }
    
Field Type Description
path string Field path to an array field. To specify a field path, prefix the field name with a dollar sign $ and enclose in quotes.
includeArrayIndex string Optional. The name of a new field to hold the array index of the element. The name cannot start with a dollar sign $.
preserveNullAndEmptyArrays boolean

Optional. If true, if the path is null, missing, or an empty array, $unwind outputs the document. If false, $unwind does not output a document if the path is null, missing, or an empty array.

The default value is false.

Behaviors

Non-Array Field Path

Changed in version 3.2: $unwind stage no longer errors on non-array operands. If the operand does not resolve to an array but is not missing, null, or an empty array, $unwind treats the operand as a single element array.

Previously, if a value in the field specified by the field path is not an array, db.collection.aggregate() generates an error.

Missing Field

If you specify a path for a field that does not exist in an input document or the field is an empty array, $unwind, by default, ignores the input document and will not output documents for that input document.

New in version 3.2: To output documents where the array field is missing, null or an empty array, use the option preserveNullAndEmptyArrays.

Examples

Unwind Array

Consider an inventory with the following document:

{ "_id" : 1, "item" : "ABC1", sizes: [ "S", "M", "L"] }

The following aggregation uses the $unwind stage to output a document for each element in the sizes array:

db.inventory.aggregate( [ { $unwind : "$sizes" } ] )

The operation returns the following results:

{ "_id" : 1, "item" : "ABC1", "sizes" : "S" }
{ "_id" : 1, "item" : "ABC1", "sizes" : "M" }
{ "_id" : 1, "item" : "ABC1", "sizes" : "L" }

Each document is identical to the input document except for the value of the sizes field which now holds a value from the original sizes array.

includeArrayIndex and preserveNullAndEmptyArrays

New in version 3.2.

A collection inventory has the following documents:

{ "_id" : 1, "item" : "ABC", "sizes": [ "S", "M", "L"] }
{ "_id" : 2, "item" : "EFG", "sizes" : [ ] }
{ "_id" : 3, "item" : "IJK", "sizes": "M" }
{ "_id" : 4, "item" : "LMN" }
{ "_id" : 5, "item" : "XYZ", "sizes" : null }

The following $unwind operations are equivalent and return a document for each element in the sizes field. If the sizes field does not resolve to an array but is not missing, null, or an empty array, $unwind treats the non-array operand as a single element array.

db.inventory.aggregate( [ { $unwind: "$sizes" } ] )
db.inventory.aggregate( [ { $unwind: { path: "$sizes" } } ] )

The operation returns the following documents:

{ "_id" : 1, "item" : "ABC", "sizes" : "S" }
{ "_id" : 1, "item" : "ABC", "sizes" : "M" }
{ "_id" : 1, "item" : "ABC", "sizes" : "L" }
{ "_id" : 3, "item" : "IJK", "sizes" : "M" }

The following $unwind operation uses the includeArrayIndex option to output also the array index of the array element.

db.inventory.aggregate( [ { $unwind: { path: "$sizes", includeArrayIndex: "arrayIndex" } } ] )

The operation unwinds the sizes array and includes the array index of the array index in the new arrayIndex field. If the sizes field does not resolve to an array but is not missing, null, or an empty array, the arrayIndex field is null.

{ "_id" : 1, "item" : "ABC", "sizes" : "S", "arrayIndex" : NumberLong(0) }
{ "_id" : 1, "item" : "ABC", "sizes" : "M", "arrayIndex" : NumberLong(1) }
{ "_id" : 1, "item" : "ABC", "sizes" : "L", "arrayIndex" : NumberLong(2) }
{ "_id" : 3, "item" : "IJK", "sizes" : "M", "arrayIndex" : null }

The following $unwind operation uses the preserveNullAndEmptyArrays option to include in the output those documents where sizes field is missing, null or an empty array.

db.inventory.aggregate( [
   { $unwind: { path: "$sizes", preserveNullAndEmptyArrays: true } }
] )

In addition to unwinding the documents where the sizes is an array of elements or a non-null, non-array field, the operation outputs, without modification, those documents where the sizes field is missing, null or an empty array:

{ "_id" : 1, "item" : "ABC", "sizes" : "S" }
{ "_id" : 1, "item" : "ABC", "sizes" : "M" }
{ "_id" : 1, "item" : "ABC", "sizes" : "L" }
{ "_id" : 2, "item" : "EFG" }
{ "_id" : 3, "item" : "IJK", "sizes" : "M" }
{ "_id" : 4, "item" : "LMN" }
{ "_id" : 5, "item" : "XYZ", "sizes" : null }