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

The array provided to $arrayToObject must be in one of the two following formats:

• An array of two-element arrays where the first element is the field name, and the second element is the field value.
• An array of documents that contain a k field and a v field, where the k field contains the field name and the v field contains the value.

Format 1

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

{
	"_id" : 1,
	"data" : [
		[
			"name",
			"Fetch"
		],
		[
			"type",
			"Dog"
		]
	]
}

We can use the $arrayToObject operator to return the data field as a document object:

db.test.aggregate(
   [
     { $match: { _id: { $in: [ 1 ] } } },
     { $project: { 
        _id: 0,
        result: { $arrayToObject: "$data" } } 
         }
   ]
)

Result:

{ "result" : { "name" : "Fetch", "type" : "Dog" } }

Format 2

Suppose we have a document like this:

{
	"_id" : 2,
	"data" : [
		{
			"k" : "name",
			"v" : "Fetch"
		},
		{
			"k" : "type",
			"v" : "Dog"
		}
	]
}

In this case, the k fields contain the keys and the v fields contain the values.

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

db.test.aggregate(
   [
     { $match: { _id: { $in: [ 2 ] } } },
     { $project: { 
        _id: 0,
        result: { $arrayToObject: "$data" } } 
         }
   ]
)

Result:

{ "result" : { "name" : "Fetch", "type" : "Dog" } }

We can see that this resulted in the same document that was produced in the previous example.

Non-Conforming Arrays

The argument provided to $arrayToObject can be any valid expression as long as it resolves to an array of two-element arrays or array of documents that contains k and v fields.

If the argument does not adhere to this, an error occurs.

Suppose we have the following document:

{ "_id" : 3, "data" : [ [ "name", "Fetch", "Dog" ] ] }

This array contains three elements.

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

db.test.aggregate(
   [
     { $match: { _id: { $in: [ 3 ] } } },
     { $project: { 
        _id: 0,
        result: { $arrayToObject: "$data" } } 
         }
   ]
)

Result:

uncaught exception: Error: command failed: {
	"ok" : 0,
	"errmsg" : "$arrayToObject requires an array of size 2 arrays,found array of size: 3",
	"code" : 40397,
	"codeName" : "Location40397"
} : 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, $arrayToObject requires an array of size 2 arrays.

Here’s another document that contains a non-conforming array:

{ "_id" : 4, "data" : [ { "a" : "name", "b" : "Fetch" } ] }

In this case, the document within the array uses a and b fields instead of k and v.

Here’s what happens when we apply $arrayToObject:

db.test.aggregate(
   [
     { $match: { _id: { $in: [ 4 ] } } },
     { $project: { 
        _id: 0,
        result: { $arrayToObject: "$data" } } 
         }
   ]
)

Result:

uncaught exception: Error: command failed: {
	"ok" : 0,
	"errmsg" : "$arrayToObject requires an object with keys 'k' and 'v'. Missing either or both keys from: {a: \"name\", b: \"Fetch\"}",
	"code" : 40393,
	"codeName" : "Location40393"
} : 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

In this case, the error states that $arrayToObject requires an object with keys 'k' and 'v'.

Wrong Type

Similarly, if the argument is not even an array, an error occurs.

Suppose we have the following document:

{ "_id" : 5, "data" : "None" }

The data field contains a string.

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

db.test.aggregate(
   [
     { $match: { _id: { $in: [ 5 ] } } },
     { $project: { 
        _id: 0,
        result: { $arrayToObject: "$data" } } 
         }
   ]
)

Result:

uncaught exception: Error: command failed: {
	"ok" : 0,
	"errmsg" : "$arrayToObject requires an array input, found: string",
	"code" : 40386,
	"codeName" : "Location40386"
} : 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, $arrayToObject requires an array input.

Null Values

Providing null results in null.

Suppose we have the following document:

{ "_id" : 6, "data" : null }

And we apply $arrayToObject:

db.test.aggregate(
   [
     { $match: { _id: { $in: [ 6 ] } } },
     { $project: { 
        _id: 0,
        result: { $arrayToObject: "$data" } } 
         }
   ]
)

Result:

{ "result" : null }

Missing Fields

If the field is missing, the result is null.

Suppose we have the following document:

{ "_id" : 7 }

And we apply $arrayToObject:

db.test.aggregate(
   [
     { $match: { _id: { $in: [ 7 ] } } },
     { $project: { 
        _id: 0,
        result: { $arrayToObject: "$data" } } 
         }
   ]
)

Result:

{ "result" : null }

Repeating Field Names

According to the MongoDB documentation, 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.