Field Paths

Nested Fields

The following example uses the planets collection from the Atlas Sample Databases. Each document in this collection has the following structure:

{
 _id: new ObjectId("6220f6b78a733c51b416c80e"),
 name: "Uranus",
 orderFromSun: 7,
 hasRings: true,
 mainAtmosphere: [ "H2", "He", "CH4" ],
 surfaceTemperatureC: { min: null, max: null, mean: -197.2 }
}

To specify the nested field mean within the surfaceTemperatureC field, use dot notation ("field.nestedField") with a dollar sign $. The following aggregation pipeline projects only the mean nested field value for each document:

db.planets.aggregate( [
 {
 $project: {
 nested_field: "$surfaceTemperatureC.mean"
 }
 }
] )

Below is an example returned document:

{ _id: ObjectId('6220f6b78a733c51b416c80e'), nested_field: -197.2 }

Array of Nested Fields

You can use dot notation in a field path to access a field that is nested within an array.

For example, consider a products collection that contains an instock field. The instock field contains an array of nested warehouse fields.

db.products.insertMany( [
 { item: "journal", instock: [ { warehouse: "A"}, { warehouse: "C" } ] },
 { item: "notebook", instock: [ { warehouse: "C" } ] },
 { item: "paper", instock: [ { warehouse: "A" }, { warehouse: "B" } ] },
 { item: "planner", instock: [ { warehouse: "A" }, { warehouse: "B" } ] },
 { item: "postcard", instock: [ { warehouse: "B" }, { warehouse: "C" } ] }
] )

The following aggregation pipeline uses $instock.warehouse to access the nested warehouse fields.

db.products.aggregate( [
 {
 $project: {
 item: 1,
 warehouses: "$instock.warehouse"
 }
 }
] )

In this example, $instock.warehouse outputs an array of values that are in the nested warehouse field for each document. The pipeline returns the following documents:

[
 {
 _id: ObjectId('6740b55e33b29cf6b1d884f7'),
 item: "journal",
 warehouses: [ "A", "C" ]
 },
 {
 _id: ObjectId('6740b55e33b29cf6b1d884f8'),
 item: "notebook",
 warehouses: [ "C" ]
 },
 {
 _id: ObjectId('6740b55e33b29cf6b1d884f9'),
 item: "paper",
 warehouses: [ "A", "B" ]
 },
 {
 _id: ObjectId('6740b55e33b29cf6b1d884fa'),
 item: "planner",
 warehouses: [ "A", "B" ]
 },
 {
 _id: ObjectId('6740b55e33b29cf6b1d884fb'),
 item: "postcard",
 warehouses: [ "B", "C" ]
 }
]

Array of Nested Arrays

You can also use dot notation with a dollar sign $ in a field path to access an array within a nested array.

This example uses a fruits collection that contains the following document:

db.fruits.insertOne(
 {
 _id: ObjectId("5ba53172ce6fa2fcfc58e0ac"),
 inventory: [
 {
 apples: [
 "macintosh",
 "golden delicious",
 ]
 },
 {
 oranges: [
 "mandarin",
 ]
 },
 {
 apples: [
 "braeburn",
 "honeycrisp",
 ]
 }
 ]
 }
)

The document in the collection contains an inventory array where each element in the array is an object that contains a nested array field.

Consider the following aggregation pipeline:

db.fruits.aggregate( [
 { $project:
 { all_apples: "$inventory.apples" } }
] )

In this pipeline, $inventory.apples resolves to an array of nested arrays. The pipeline returns the following document:

{
 _id: ObjectId('5ba53172ce6fa2fcfc58e0ac'),
 all_apples: [
 [ "macintosh", "golden delicious" ],
 [ "braeburn", "honeycrisp" ]
 ]
}