$arrayToObject (aggregation)
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Definition
$arrayToObject
New in version 3.4.4.
Converts an array into a single document; the array must be either:
An array of two-element arrays where the first element is the field name, and the second element is the field value:
[ [ [ "item", "abc123" ], [ "qty", 25 ] ] ]
- OR -
An array of documents that contains two fields,
k
andv
where:The
k
field contains the field name.The
v
field contains the value of the field.
[ [ { "k": "item", "v": "abc123" }, { "k": "qty", "v": 25 } ] ]
$arrayToObject
has the following syntax:{ $arrayToObject: <expression> } The
<expression>
can be any valid expression that resolves to an array of two-element arrays or array of documents that contains "k" and "v" fields.For more information on expressions, see Expressions.
Behavior
If the name of a field repeats in the array,
Starting in 4.0.5,
$arrayToObject
uses 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,
$arrayToObject
uses 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,
$arrayToObject
uses 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:
{ "_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 a inventory
collection with the following documents:
{ "_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 } }