$objectToArray (aggregation)
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Definition
$objectToArray
New in version 3.4.4.
Converts a document to an array. The return array contains an element for each field/value pair in the original document. Each element in the return array is a document that contains two fields
k
andv
:The
k
field contains the field name in the original document.The
v
field contains the value of the field in the original document.
$objectToArray
has the following syntax:{ $objectToArray: <object> } The
<object>
expression can be any valid expression as long as it resolves to a document object.$objectToArray
applies to the top-level fields of its argument. If the argument is a document that itself contains embedded document fields, the$objectToArray
does not recursively apply to the embedded document fields.For more information on expressions, see Expressions.
Behavior
For more information on expressions, see Expressions.
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Examples
$objectToArray
Example
Consider a inventory
collection with the following documents:
{ "_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" : "XYZ1", dimensions: { l: 70, w: 75, uom: "cm" } }
The following aggregation pipeline operation use the
$objectToArray
to return the dimensions
field as an array:
db.inventory.aggregate( [ { $project: { item: 1, dimensions: { $objectToArray: "$dimensions" } } } ] )
The operation returns the following:
{ "_id" : 1, "item" : "ABC1", "dimensions" : [ { "k" : "l", "v" : 25 }, { "k" : "w", "v" : 10 }, { "k" : "uom", "v" : "cm" } ] } { "_id" : 2, "item" : "ABC2", "dimensions" : [ { "k" : "l", "v" : 50 }, { "k" : "w", "v" : 25 }, { "k" : "uom", "v" : "cm" } ] } { "_id" : 3, "item" : "XYZ1", "dimensions" : [ { "k" : "l", "v" : 70 }, { "k" : "w", "v" : 75 }, { "k" : "uom", "v" : "cm" } ] }
$objectToArray
to Sum Nested Fields
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 uses the
$objectToArray
along with $unwind
and
$group
to calculate the total items in stock per warehouse.
db.inventory.aggregate([ { $project: { warehouses: { $objectToArray: "$instock" } } }, { $unwind: "$warehouses" }, { $group: { _id: "$warehouses.k", total: { $sum: "$warehouses.v" } } } ])
The operation returns the following:
{ "_id" : "warehouse3", "total" : 200 } { "_id" : "warehouse2", "total" : 1000 } { "_id" : "warehouse1", "total" : 2500 }
$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 } }