Field Path Operand

You can pass the array field path to $unwind. When using this syntax, $unwind does not output a document if the field value is null, missing, or an empty array.

{ $unwind: <field path> }

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

Document Operand with Options

You can pass a document to $unwind to specify various behavior options.

{
  $unwind:
    {
      path: <field path>,
      includeArrayIndex: <string>,
      preserveNullAndEmptyArrays: <boolean>
    }
}

Non-Array Field Path

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.

Unwind Array

From the mongo shell, create a sample collection named inventory with the following document:

db.inventory.insertOne({ "_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

From the mongo shell, create a sample collection named inventory2 with the following documents:

db.inventory2.insertMany([
  { "_id" : 1, "item" : "ABC", price: NumberDecimal("80"), "sizes": [ "S", "M", "L"] },
  { "_id" : 2, "item" : "EFG", price: NumberDecimal("120"), "sizes" : [ ] },
  { "_id" : 3, "item" : "IJK", price: NumberDecimal("160"), "sizes": "M" },
  { "_id" : 4, "item" : "LMN" , price: NumberDecimal("10") },
  { "_id" : 5, "item" : "XYZ", price: NumberDecimal("5.75"), "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.inventory2.aggregate( [ { $unwind: "$sizes" } ] )
db.inventory2.aggregate( [ { $unwind: { path: "$sizes" } } ] )

The operation returns the following documents:

{ "_id" : 1, "item" : "ABC", "price" : NumberDecimal("80"), "sizes" : "S" }
{ "_id" : 1, "item" : "ABC", "price" : NumberDecimal("80"), "sizes" : "M" }
{ "_id" : 1, "item" : "ABC", "price" : NumberDecimal("80"), "sizes" : "L" }
{ "_id" : 3, "item" : "IJK", "price" : NumberDecimal("160"), "sizes" : "M" }

db.inventory2.aggregate( [
  {
    $unwind:
      {
        path: "$sizes",
        includeArrayIndex: "arrayIndex"
      }
   }])

{ "_id" : 1, "item" : "ABC", "price" : NumberDecimal("80"), "sizes" : "S", "arrayIndex" : NumberLong(0) }
{ "_id" : 1, "item" : "ABC", "price" : NumberDecimal("80"), "sizes" : "M", "arrayIndex" : NumberLong(1) }
{ "_id" : 1, "item" : "ABC", "price" : NumberDecimal("80"), "sizes" : "L", "arrayIndex" : NumberLong(2) }
{ "_id" : 3, "item" : "IJK", "price" : NumberDecimal("160"), "sizes" : "M", "arrayIndex" : null }

db.inventory2.aggregate( [
   { $unwind: { path: "$sizes", preserveNullAndEmptyArrays: true } }
] )

{ "_id" : 1, "item" : "ABC", "price" : NumberDecimal("80"), "sizes" : "S" }
{ "_id" : 1, "item" : "ABC", "price" : NumberDecimal("80"