Definition
$arrayToObjectConverts an array into a single document.
$arrayToObjecthas the following syntax:{ $arrayToObject: <expression> }The
<expression>can be any valid expression that resolves to:An array of two-element arrays where the first element is the field name, and the second element is the field value:
$arrayToObject: [ [ [ "item", "abc123" ], [ "qty", 25 ] ]]
- OR -
An array of documents that contains two fields,
kandvwhere:The
kfield contains the field name.The
vfield contains the value of the field.
$arrayToObject: [ [ { "k": "item", "v": "abc123" }, { "k": "qty", "v": 25 } ]]
Behavior
If you pass in an empty array to $arrayToObject, MongoDB creates an empty object.
If the name of a field repeats in the array,
Starting in 4.0.5,
$arrayToObjectuses the last value for that field. For 4.0.0-4.0.4, the value used depends on the driver.Starting in 3.6.10,
$arrayToObjectuses the last value for that field. For 3.6.0-3.6.9, the value used depends on the driver.Starting in 3.4.19,
$arrayToObjectuses the last value for that field. For 3.4.0-3.4.19, the value uses depends on the driver.
Example | Results |
|---|---|
| Given a document with the field |
| |
| Starting in versions 4.0.5+ (3.6.10+ and 3.4.19+), if the name of a field repeats in the array, |
Examples
$arrayToObject Example
Consider a inventory collection with the following documents:
db.inventory.insertMany( [ { _id: 1, item: "ABC1", dimensions: [ { "k": "l", "v": 25} , { "k": "w", "v": 10 }, { "k": "uom", "v": "cm" } ] }, { _id: 2, item: "ABC2", dimensions: [ [ "l", 50 ], [ "w", 25 ], [ "uom", "cm" ] ] }, { _id: 3, item: "ABC3", dimensions: [ [ "l", 25 ], [ "l", "cm" ], [ "l", 50 ] ] }] )The following aggregation pipeline operation use the $arrayToObject to return the dimensions field as a document:
db.inventory.aggregate( [ { $project: { item: 1, dimensions: { $arrayToObject: "$dimensions" } } } ])The operation returns the following:
{ _id: 1, item: "ABC1", dimensions: { "l" : 25, "w" : 10, "uom" : "cm" } }{ _id: 2, item: "ABC2", dimensions: { "l" : 50, "w" : 25, "uom" : "cm" } }{ _id: 3, item: "ABC3", dimensions: { "l" : 50 } }Starting in versions 4.0.5+ (3.6.10+ and 3.4.19+), if the name of a field repeats in the array, $arrayToObject uses the last value for that field.
$objectToArray + $arrayToObject Example
Consider an inventory collection with the following documents:
db.inventory.insertMany( [ { _id: 1, item: "ABC1", instock: { warehouse1: 2500, warehouse2: 500 } }, { _id: 2, item: "ABC2", instock: { warehouse2: 500, warehouse3: 200}, }] )The following aggregation pipeline operation calculates the total in stock for each item and adds to the instock document:
db.inventory.aggregate( [ { $addFields: { instock: { $objectToArray: "$instock" } } }, { $addFields: { instock: { $concatArrays: [ "$instock", [ { "k": "total", "v": { $sum: "$instock.v" } } ] ] } } } , { $addFields: { instock: { $arrayToObject: "$instock" } } }] )The operation returns the following:
{ _id: 1, item: "ABC1", instock: { warehouse1: 2500, warehouse2: 500, total: 3000 } }{ _id: 2, item: "ABC2", instock: { warehouse2: 500, warehouse3: 200, total: 700 } }