In MongoDB, the $objectToArray aggregation pipeline operator converts a document to an array.

The array produced by $objectToArray contains an element for each field/value pair in the original document. Each element is a document that contains a k field and a v field:

• The k field contains the field name in the original document.
• The v field contains the value of the field in the original document.

Example

Suppose we have a collection called dogs with the following document:

{
	"_id" : 1,
	"name" : "Fetch",
	"specs" : {
		"height" : 400,
		"weight" : 55,
		"color" : "brown"
	}
}

We can use the $objectToArray operator to return the specs field as an array:

db.dogs.aggregate(
   [
     { $match: { _id: { $in: [ 1 ] } } },
     { $project: { 
        _id: 0,
        result: { $objectToArray: "$specs" } } 
         }
   ]
).pretty()

Result:

{
	"result" : [
		{
			"k" : "height",
			"v" : 400
		},
		{
			"k" : "weight",
			"v" : 55
		},
		{
			"k" : "color",
			"v" : "brown"
		}
	]
}

Nested Documents

The $objectToArray operator only applies to the top level field. It doesn’t recursively apply to any embedded documents.

Suppose we have a document like this:

{
	"_id" : 2,
	"name" : "Wag",
	"specs" : {
		"height" : 50,
		"weight" : 5,
		"color" : {
			"eyes" : "brown",
			"coat" : "black"
		}
	}
}

Here’s what happens when we apply $objectToArray to that document:

db.dogs.aggregate(
   [
     { $match: { _id: { $in: [ 2 ] } } },
     { $project: { 
        _id: 0,
        result: { $objectToArray: "$specs" } } 
         }
   ]
).pretty()

Result:

{
	"result" : [
		{
			"k" : "height",
			"v" : 50
		},
		{
			"k" : "weight",
			"v" : 5
		},
		{
			"k" : "color",
			"v" : {
				"eyes" : "brown",
				"coat" : "black"
			}
		}
	]
}

In this case, the top level document is converted to the k/v format, but the embedded document remains the same as in the original document.

Wrong Type

The argument provided to $objectToArray can be any valid expression as long as it resolves to a document object.

If the argument does not resolve to a document object, an error occurs.

Suppose we have the following document:

{ "_id" : 3, "name" : "Fetch", "specs" : "None" }

The specs field contains a string.

Here’s what happens when we apply $objectToArray to that document:

db.dogs.aggregate(
   [
     { $match: { _id: { $in: [ 3 ] } } },
     { $project: { 
        _id: 0,
        result: { $objectToArray: "$specs" } } 
         }
   ]
)

Result:

uncaught exception: Error: command failed: {
	"ok" : 0,
	"errmsg" : "$objectToArray requires a document input, found: string",
	"code" : 40390,
	"codeName" : "Location40390"
} : aggregate failed :
_getErrorWithCode@src/mongo/shell/utils.js:25:13
doassert@src/mongo/shell/assert.js:18:14
_assertCommandWorked@src/mongo/shell/assert.js:639:17
assert.commandWorked@src/mongo/shell/assert.js:729:16
DB.prototype._runAggregate@src/mongo/shell/db.js:266:5
DBCollection.prototype.aggregate@src/mongo/shell/collection.js:1058:12
@(shell):1:1

As the error states, $objectToArray requires a document input.

Null Values

Providing null results in null.

Suppose we have the following document:

{ "_id" : 4, "name" : "Fetch", "specs" : null }

And we apply $objectToArray:

db.dogs.aggregate(
   [
     { $match: { _id: { $in: [ 4 ] } } },
     { $project: { 
        _id: 0,
        result: { $objectToArray: "$specs" } } 
         }
   ]
)

Result:

{ "result" : null }

Missing Fields

If the field is missing, the result is null.

Suppose we have the following document:

{ "_id" : 5, "name" : "Fetch" }

And we apply $objectToArray:

db.dogs.aggregate(
   [
     { $match: { _id: { $in: [ 5 ] } } },
     { $project: { 
        _id: 0,
        result: { $objectToArray: "$specs" } } 
         }
   ]
)

Result:

{ "result" : null }